Hidden fields let you collect and use data without showing it to respondents. They help track users, personalize forms, and pass data between systems.
Hidden fields are invisible to respondents but allow you to gather and use data effectively.
What You Can Do with Hidden Fields
Common use cases include:
Track where respondents came from (social media, website, campaigns)
Pass existing respondent details (e.g., name or email)
Personalize forms (e.g., greet users by name)
Prefill form inputs automatically
Pass information between forms
Pass data from Formly to other applications (e.g., send respondent details to Calendly, CRM systems, or other tools)
Track marketing campaigns using UTM parameters
How to Create a Hidden Field
1. Open your form from your workspace. Towards the top of your screen, click Logic tab. Then click Hidden fields.
IMAGE coming soon
2. Select an option from the dropdown, or enter a custom value. You can add as many hidden fields as you like.
In this example, we will add custom field 'first_name' for respondents name and select one default value: 'utm_source' for traffic source.
IMAGE coming soon
Note!
1. Hidden fields must be written in lower-case letters and use underscores for spaces.
2. You can't use the "name" URL parameter in forms embedded on Wordpress sites because it is a reserved term on Wordpress. You can bypass this restriction by using the "first_name" or "last_name" URL parameters in your form instead.
3. Click on Save
IMAGE
Your form URL should look like this. Replace XXX with the appropriate values.
https://form.getformly.com/f/12?first_name=XXX&utm_source=XXX
4. Complete your form and Publish it. Find your form’s URL in the Share tab.
IMAGE
When you are ready to share the form, add the parameters and corresponding values to your URL before sharing the link. There are two ways to add the parameters:
Manually: you manually add information to the form URL parameters before sharing your form.
Automated: you can use your CRM system (such as Salesforce) or email automation tool (such as Mailchimp or Acumbamail) to populate your data, so your respondents automatically receive a customized URL with the correct data.
Note! The first hidden field should start with ? . When adding multiple fields, separate them with & .
For example, if your respondent’s first name is Maria and the traffic source is Linkedin, the URL would look like this:
https://form.getformly.com/f/12?first_name=Maria&utm_source=Linkedin
Viewing Hidden Field Data
In your Responses tab, you can see each individual submission and its hidden fields data.
IMAGE coming soon
Give it a try. Copy this link, paste it in a web browser, and replace the “XXX” with your name and an imagined source, like ‘Linkedin’. Then hit ‘enter’ and see what the form does.
You can pass text in the URL parameter with a space. This can come in handy when passing, for instance, full names, like “Maria Smith”. You need to replace the spaces with “%20” in the URL for the URL parameter to appear with the space in your form. So your URL would look like this:
Note! If you edit the name of a hidden field that has already been collecting responses, the previously collected responses will also be deleted.
Examples
1. Track the traffic source of your respondent
You can track the traffic source of your respondent (social media, triggers on your website, or other channels) as explained above.
This helps you measure which channels drive the most responses.
2. Hidden fields in an embedded form
You can add hidden fields to forms that are embedded on your website.
This is useful for tracking which page the form was completed on.
For example, if your form appears on multiple pages:
Home page:
https://form.getformly.com/f/12?utm_source=homepage
About page:
https://form.getformly.com/f/12?utm_source=about
You'll then be able to see in your Responses how many respondents answered your form on your homepage or your About page.
3. Pre-populate form fields
Add a default answer
You can pre-fill input fields with text or numerical values. If a respondent submits your form without changing the pre-populated answers, these will be included in their form response.
To add a default answer, go to Question settings in the right panel and type a custom default answer or choose the value you want to pre-fill from the dropdown menu.
You can insert data from:
Hidden fields
Variables
Responses from previous questions
Note! Default answer is available for below seven fields:
Short Text
Long Text
Number
Website
Rating
Opinion Scale
Example: Pre-populate name, email and rating using hidden fields
VIDEO coming soon
4. Populate hidden fields with Mailchimp and other tools
Email platforms like Mailchimp use merge tags to insert personalized contact data into emails.
When a recipient clicks a form link containing merge tags, the tags are replaced with stored contact information and passed into hidden fields.
This allows you to:
personalize forms
pass contact details automatically
avoid manual URL customization
4.1 Create a form with hidden fields
Create a form and add the hidden fields you need. In this example we want to include the email address and the source of the respondent.
IMAGE coming soon
Publish your form and go to the Share tab to copy the form URL.
4.2 Create a Mailchimp campaign
Create a campaign and embed a Button in your email. In the Link to: Web address field add the form URL including the hidden field URL parameters.
Note! The first hidden field should be preceded by ? . When adding multiple fields, separate them with & . For example:
https://form.getformly.com/f/12345?email=*|EMAIL|*&source=email
IMAGE coming soon
In this example we match the hidden field ‘email’ to the email merge tag in Mailchimp (*|EMAIL|*), and the hidden field ‘source’ gets the fixed value ‘email’ (because the source of all respondents will be email).
4.3 Launch the campaign
When you launch your email campaign the form URL will be automatically updated with the content in the corresponding audience field, and the hidden field values will be available in your form results.
A similar process works with other email and CRM platforms — including HubSpot, ActiveCampaign, Acumbamail, and any system that supports merge tags or personalization tokens.
5. Pass information from one form to another
You can connect forms and reuse responses using:
Hidden fields
URL parameters
Redirect to URL
This allows answers from one form to appear in another, creating a seamless multi-step workflow.
VIDEO coming soon
6. Pass data from Formly to other applications
Hidden fields can pass respondent details to external tools.
Example:
Send a respondent’s Name and Email to Calendly so they don’t need to re-enter their details.
This improves user experience and reduces friction.
The same method works with other tools that accept URL parameters.
VIDEO coming soon
7. Redirect to different URLs
Set dynamic content in the URL (variables, hidden fields, answers from question, and Ending screens) so you can automatically redirect user to different URL based on scores, options selected etc.