The Prefill API in the Formstack Forms 2025 API allows you to securely pre-populate fields in a form by generating a tokenized link. This enables forms to open with information already filled in, without exposing any data in the URL. Using the Prefill API, you can programmatically insert values into form fields and provide a seamless experience for users who need to review, update, or confirm information.
Explore the API Documentation
To implement the Prefill API, refer to the detailed endpoint and request specifications in our developer documentation:
👉 Formstack Forms 2025 API – Prefill Documentation
This resource includes authentication instructions and outlines the required parameters needed to build a Prefill API request.
Important note: When using the Prefill API with complex fields such as Name, Address or other multi-part fields, you will need to provide values using the correct internal subfield structure. Since these subfield keys are not automatically shown in the API, developers may need to review the field schema to determine the proper structure.The Prefill API will still process these values correctly, but setting up complex fields may require extra configuration compared to simple fields.
Why Use the Prefill API?
The Prefill API provides a secure method for inserting data into form fields before a user opens the form. Because all prefilling is handled through an authenticated API request, sensitive data is never visible in the URL or browser.
This makes the Prefill API useful for scenarios such as:
- Prefilling forms with known user or account data
- Reducing repetitive data entry
- Passing customer, patient, or applicant information securely
- Supporting personalized onboarding or follow-up workflows
- Integrating forms into existing backend systems
How the Prefill API Works (High-Level Overview)
The Prefill API is available in the Formstack Forms 2025 API under the /prefill endpoint.
- Identify the form using its Form ID.
- Send a POST request specifying the fields to prefill and their values.
- Receive a secure, tokenized form link representing those values (not exposing them).
- Open the form, and the fields will automatically appear pre-populated.
Example Prefill Behavior
For a form collecting customer or account details, you can retrieve known values from your system, send them to the Prefill endpoint, and receive a secure form link with all fields filled in for the user, without exposing any sensitive information in the URL.
Benefits of Using the Prefill API
Using the Prefill API helps you:
- Protect sensitive data, since nothing appears in the URL
- Save time for your users by reducing repeated data entry
- Deliver a smoother form experience, especially when users need to review or confirm known information
- Integrate forms into your existing systems, allowing data to flow automatically
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Maintain consistent workflows, since each prefilled form link is securely generated through the API