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Prefilling removes friction. By passing known data (like a user’s name, email, or marketing UTM tags) in the URL, you increase conversion rates and ensure data accuracy. Prefill runs only on embed and popup fills — not on the plain public fill link.

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How Prefilling Works

Flex Forms reads the URL query string when an embed or popup form loads. If a query parameter matches a field’s exact Name property, that field is prefilled.
Example embed URL
1

Identify the Field Name

Open the Studio, select a field, and check its Name in the Inspector panel. Note: Use the internal Name, not the visual Label.
2

Append to an embed or popup URL

Add ?fieldname=value to the embed or popup URL (Share → Embed), not the standalone fill link.
3

Lock the Field (Optional)

If you don’t want the user to change the prefilled value (e.g., a hidden CRM ID), change the field’s visibility to Hidden or set it to Read-Only in the Studio settings.
Need prefill on a shared link? Use a Standard embed and put the query string on the embed iframe URL. The plain fill link ignores prefill params by design.

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Security Best Practices

Never pass sensitive secrets (like passwords or secure tokens) in the query string. URLs are stored in browser history and intermediate proxy logs. If you need to pass secure context, use an opaque ID and resolve it via your backend integrations.
Last modified on September 7, 2026