Booking Calendar URL Parameters: When To Use The C_ID, External URL Parameters or Both

Avoid asking your guests for information you already have.

Use the c_id parameter to identify an existing OnceHub Contact, external URL parameters to prefill contact information and other booking form questions from an outside source, or both together when you are not sure whether a Contact is in OnceHub.

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Which Approach Fits You Best?

Your guests’ contact details are all stored in OnceHub

Use c_id on its own: The link carries only an identifier, so your guests’ details are not displayed in the URL, which makes this the more secure approach. OnceHub already holds their answers, so it skips those questions and asks only for what’s required and missing. If it has everything, the guest doesn’t see the booking form at all.

For example:

• Sending a personalized link to an existing Contact via email or campaign: https://oncehub.com/example?c_id=CTC-xxxxxxxxxxxxxx
• Using their stored email address: https://oncehub.com/example?c_id=john.doe@example.com

Your guests’ contact details are not in OnceHub

Use external URL parameters on their own: Your link carries the details themselves, so they are visible in the URL. They appear prefilled for the guest to review, and skip=1 skips the form when everything required is filled in.

For example, prefilling guest details from an external CRM and skipping the form step:

https://oncehub.com/example?name=John%20Doe&email=john.doe@example.com&skip=1

You’re not sure where your guests’ contact details live

Use both in the same link: One link works for everyone. OnceHub uses the c_id to identify the Contact (if it is in OnceHub) and skips what it already knows. The external URL parameters cover those Contacts who are not in OnceHub.

For example, send an email broadcast using CRM placeholders to handle both existing Contacts and new leads:

https://oncehub.com/example?c_id={contact_email}&name={contact_name}&email={contact_email}

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How Skipping Works

Skipping works at two levels, and the two approaches use different methods:

• When using c_id (question-level skipping): OnceHub skips each booking form question that is mapped to a Contact Property that already has a saved value. This happens automatically. If every required question is skipped, the entire booking form is skipped automatically.
• When using external URL parameters: The skip=1 parameter explicitly instructs OnceHub to skip the entire booking form step. It only takes effect when all mandatory questions are prefilled through the URL. Without the skip=1 parameter, the booking form is displayed even if all the data is provided.

Note: If the SMS number question on your booking form is prefilled, the booking form can still be skipped automatically. Since the guest doesn’t interact with the booking form, SMS opt-in consent won’t be collected and will remain false.

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Option 1: The c_id (Contact ID) Parameter

The c_id parameter uses either a unique Contact ID (for example, c_id=CTC-xxxxxxxxxxxxxx) or the email address stored on an existing Contact (for example, c_id=john@example.com) to associate the booking with that Contact. The link carries only this identifier, so your guests’ details are not displayed in the URL.

Prerequisites

For OnceHub to skip booking form questions for an existing Contact:

• Your booking form questions must be mapped to the corresponding Contact Properties. See Mapping Booking Calendar Questions to Object Properties.
• The mapped Contact Properties must contain values in the Contact record.

When to Use

Use c_id when you are scheduling with existing OnceHub Contacts, for example by sending a personalized booking link manually or in an email campaign. Guests book without re-entering information you already hold.

How It Works

Append a Contact ID or stored email address to your booking link. You can find the Contact ID on the Contact’s detail page. See The Contact Detail Page for more information.

https://oncehub.com/example?c_id=CTC-xxxxxxxxxxxxxx

https://oncehub.com/example?c_id=john@example.com

OnceHub matches the c_id value to the existing Contact and skips the questions for which your organization already has the answers.

If all required mapped questions have values, OnceHub automatically skips the booking form. If any required information is missing, only those questions are displayed. You don’t need to add skip=1 when you use c_id.

Data privacy safeguard: Contact information stored in OnceHub is not exposed in the URL or displayed to the guest. Because the link carries only an identifier, c_id is the more private and secure option.

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Option 2: External URL Parameters

External URL parameters pass field values directly in the booking link. The values are visible in the browser address bar and are used to prefill the corresponding booking form fields.

When to Use

Use external URL parameters when:

• Scheduling with new Contacts or capturing details live: You are collecting information from a guest who doesn’t have a Contact record in OnceHub, such as during a phone call, live chat, or an email campaign sent from an external CRM (for example, HubSpot).
• Outbound email campaigns to external leads: You are sending promotional emails or booking invites from an external marketing tool or CRM (for example, HubSpot or Mailchimp) to new prospects whose records do not exist in OnceHub, dynamically populating their details via URL merge tags.
• Allowing guests to verify information: You want guests to review or edit their details before finalizing the booking, which is the default when you don’t use skip=1.
• Passing data from external systems: Guest information lives in a third-party form, application, or CRM.

How It Works

Append the relevant parameter keys and values to the booking link:

Question / Field	URL Syntax	Description	Example
Contact Full Name	name=	Prefills the full name of the guest.	name=John%20Doe
Contact Email	email=	Prefills the email address of the guest.	email=john.doe@example.com
SMS Number	mobile_phone=	Prefills the mobile phone number of the guest.	mobile_phone=%2B17793129776

You can also prefill custom questions. Each question has its own URL parameter, which you can look up using Personalization and Tracking in the Share popup of your Booking Calendar.

Specific Parameter Processing Rules

• Phone numbers: Write the + as %2B, followed immediately by the country code and the rest of the number, without spaces or dashes. For example, for +1-779-312-9776, use mobile_phone=%2B17793129776.
• Invalid values: If a value is in an invalid or incorrect format, the question is displayed to the guest.

https://oncehub.com/example?name=John%20Doe&email=john.doe@example.com

Note: The values are visible in the browser address bar. By default, the prefilled questions remain visible so the guest can review or edit them. If any required question is empty, the guest must answer it before booking.

Skipping the Booking Form

To skip the booking form, append &skip=1. This skips the entire booking form step, but only when all mandatory questions are prefilled through URL parameters. Without it, the form is shown even when all the data is provided.

https://oncehub.com/example?name=John%20Doe&email=john.doe@example.com&skip=1

If any required fields remain empty, OnceHub displays those fields before the booking can be completed.

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Option 3: Using c_id and External URL Parameters Together

Using c_id and external URL parameters in the same link lets OnceHub recognize guests who have a Contact record, while the external URL parameter values in the link cover guests who are not in OnceHub. OnceHub skips the questions it already has answers for, and the external URL parameter values fill in the rest. The two types of URL parameters are designed to coexist in the same link and complement each other.

When to Use

Use both when:

• You aren’t sure where your guests’ details live: Some invitees have Contact records in OnceHub and some don’t.
• Your audience is a mix and you want one link for everyone: For example, you are inviting CRM Contacts, leads, and clients to the same webinar.

Prerequisites

For guests who have a Contact record, your booking form questions must be mapped to Contact Properties, and those properties must contain values in the Contact record.

How It Works

Add the Contact ID or stored email address, together with the external URL parameters, to the same booking link:

https://oncehub.com/example?c_id=CTC-xxxxxxxxxxxxxx&name=John%20Doe&email=john.doe@example.com

OnceHub skips the questions it already has answers for through c_id. The values in the link prefill the rest.

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Example: Webinar Invitation

A financial advisory firm invites its Contacts to a webinar called “Fixed income opportunities for your retirement”. The booking form asks for a name, an email, and three qualifying questions: Primary Financial Goal, Retirement Timeline, and Investible Portfolio. Each question is mapped to a Contact Property.

The firm sends the same link to everyone, using the recipient’s email address as the c_id value:

https://oncehub.com/example?c_id={contact_email}&name={contact_name}&email={contact_email}

The values in curly braces are placeholders. Your email or marketing tool replaces them with each recipient’s details when it sends the invitation. For John Doe, the link becomes:

https://oncehub.com/example?c_id=john.doe@example.com&name=John%20Doe&email=john.doe@example.com

The same link serves three types of invitee:

Invitee	What happens	What the guest sees
In your CRM but not in OnceHub	There is no matching Contact, so the name and email in the link prefill the form.	The booking form, with the name and email filled in and all three qualifying questions to answer.
A lead in OnceHub	c_id identifies the Contact, and OnceHub skips the name and email questions. A lead has no saved answers to the qualifying questions.	Only the three qualifying questions.
A Prospect, Onboarding Client, Active Client, or Former Client in OnceHub	c_id identifies the Contact, and OnceHub already has every answer.	No booking form. The webinar place is reserved.

Note: Because this link carries the name and email as values, they are visible in the URL.

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For URL parameter syntax and processing rules, see Booking Calendar URL Parameters: Syntax and Processing Rules.

To configure meeting settings or track where your bookings come from with UTM tags or hidden fields, see the corresponding Booking Calendar URL Parameters topics in the Sharing & Embedding section.