Prefill from URL parameters

URL prefill fills supported fields from the query string on the hosted or embedded page. Visible fields match on their field ID; hidden fields use paramName when it's configured.

Define stable parameter targets

const waitlist = defineForm({
  id: "waitlist",
  title: "Join the waitlist",
  pages: [{
    id: "join",
    blocks: [
      { id: "email", kind: "email", label: "Email", required: true },
      {
        id: "plan",
        kind: "select",
        label: "Plan interest",
        options: [
          { id: "team", label: "Team" },
          { id: "enterprise", label: "Enterprise" },
        ],
      },
      { id: "source", kind: "hidden", label: "Source", paramName: "src", defaultValue: "direct" },
    ],
  }],
});

Build the link with an encoder

const url = new URL("https://fillo.so/f/your-form");
url.searchParams.set("email", "ada+demo@example.com");
url.searchParams.set("plan", "team");
url.searchParams.set("src", "partner newsletter");

console.log(url.toString());

Choice values are option IDs, not labels. Multi-select takes comma-separated exact option IDs with no added spaces. Number values must be plain decimals; rating and linear scale values must be whole numbers within bounds. true, 1, and yes check a checkbox.

File upload, ranking, matrix, signature, and custom fields can't be prefilled from the URL. Invalid, empty, unknown, or out-of-range values are ignored, not inserted.

Treat the URL as respondent input

Anyone can edit a query parameter. Use prefill for convenience, routing, and attribution — never for access control, price, workspace membership, or trusted account identity.

Try it

Start with a prompt and your coding agent builds the first form, staged for your review — or open the editor.

Related

• Beta access example: Inspect a compact form with hidden fields for where signups come from.
• Prefill: Review the complete live parser contract.
• Prefill values are not appearing: Diagnose IDs, encodings, and precedence.
• Capture UTM attribution: Store campaign parameters in hidden fields.