Query params

Any query string parameter whose key matches a field’s name on the form prefills that field — no configuration needed on the form itself. This works on the embed and popup routes; the plain (non-embed) fill link does not read prefill params.

<iframe
  data-flex-form-src="https://your-app.test/forms/demo-request/embed?email=jane%40acme.com&plan=enterprise"
  loading="lazy" width="100%" height="800" frameborder="0"
  title="Demo request"
></iframe>
<script async src="https://your-app.test/vendor/flex-forms/embed.js"></script>

const url = new URL('https://your-app.test/forms/demo-request/embed');
const params = new URLSearchParams(window.location.search);
['utm_source', 'utm_medium', 'utm_campaign', 'plan'].forEach((key) => {
  if (params.has(key)) url.searchParams.set(key, params.get(key));
});
document.querySelector('iframe[data-flex-form-src]').dataset.flexFormSrc = url.toString();

The field’s name is the internal machine name you see in Studio field settings — not its display label. Only scalar (string) values are accepted; anything else is ignored.

Reserved keys

These keys always control embed behavior and are never treated as field names, even if a field happens to share the name:

Prefill values arrive as plain query-string text with no server-side validation against the field’s own rules until the respondent submits — treat prefill as a convenience, not a trusted input channel. Never rely on it to pass secrets or values you haven’t sanitized on your own page first.

Want UTM values to also show up in Insights → Visits rather than just prefilling a field? Those are captured automatically from the visit’s referrer/query string by the analytics beacon — no extra wiring needed.

Why doesn't prefill work on the plain fill link?

Prefill-from-query only runs for the embed and popup routes (isEmbed in the fill component). If you need prefill on a direct link, wrap it in a Standard embed instead.

Can I prefill a hidden field?

Yes — a field can be prefilled and simultaneously hidden via Logic, which is a common pattern for tagging submissions with a UTM source without showing it to the respondent.

Does prefill work for choice/select fields?

Yes, as long as the query value exactly matches one of the field’s option values.

Last modified on September 7, 2026

APIs & config

Extensibility

Architecture

Reserved keys

​Reserved keys

Reserved keys