URL Parameters/Hidden fields
URL parameters are a powerful way to pass information to a Fillout form to support pre-filling and personalizing the form.
Overview
URL parameters, also known as “hidden fields”, can be used to customize your form with information you already know when you’re sharing it with respondents. For example:
- Pre-filling fields with the email of the respondent
- Recording where your form was shared via a utm_source parameter (e.g. Facebook, Google, Instagram, etc.)
- Displaying the respondent's name in the form to welcome them
How to add a URL parameter or hidden field
1. Register a new parameter
Go to Settings and click
URL parameters. Then, click + Add new 
Enter a Parameter name. Here, we can tell Fillout to expect a URL Parameter with a certain name.
Note that you can optionally provide a sample value for the parameter, if you’d like to see what your form looks like with that value being used (e.g. if you have some hide/show logic based on it).
2. Use the parameter in your form
You’re all set to start using your new URL parameter now! We can now use it to:
- Pre-fill a field, by pressing the
+icon on a Default value property of any field
- Write to any integration, by heading to the Integrations at the top and creating a new connection or editing an existing one, and mapping this URL Parameter to some field like in Airtable or Notion.
- Display in plaintext in the form
3. Share your form link with the URL parameter set
To make the URL parameter active in your form, you need to set its value in the form link you share with respondents. For example, if we have a parameter called name registered, the default link would look like: https://forms.fillout.com/t/9ofaEF4d54us?name=xxxxx
Say we know that the respondent's name that we’re sending our form to, Jane. We'll type her name at the end of the link like this: https://forms.fillout.com/t/9ofaEF4d54us?name=jane
Important: When you have multiple URL parameters for one form, you’ll need to separate them using an ampersand (&), like so:
https://forms.fillout.com/t/your_form_id?email=jane@gmail.com&name=jane
Example use cases
Personalize a form with URL parameters
Type
@ in any question or field to reference information from your form. Look for URL parameters and click Continue.
Insert the parameter you have registered. 
Publish your form and share it to your respondents.
Recording URL parameters in submissions
Often, you will want to record which URL parameters were passed when submitting the form. A common scenario would be that you're pasting the link to your form on various social media platforms like Facebook, Instagram, and Linkedin. You will want to see which ones get a higher percentage of traffic. By default, Fillout stores the URL parameters used in a submission on the Results page.
If you're using an integration, and want to write your URL parameters there as well, you can either:
a. Create a Fillout field and pre-fill it with a URL parameter under Default value. Then, set it to
Hide always through the right-hand settings panel. If this field is synced to an integration already, then it will automatically write to your integration without it ever being visible to the respondent:
b. Head to your integration settings, and map the URL parameter directly. E.g. for Airtable, there is a Mappings section which you can use for this purpose. See this article for more information.