Start a form A/B test by ID. Launches the A/B setup so its variants begin serving to visitors according to their weights, moving the setup to enabled status. Starting a form A/B setup that has already been started returns a conflict.

An optional auto-close policy can be supplied in the request body. endsAt closes the test at a fixed instant and must be in the future. stopOnSignificance closes the test as soon as one variant is a statistically significant winner; on its own it is subject to a seven-day minimum runtime. Supply at least one of the two, or omit autoClose entirely for a test that never closes automatically. The policy cannot be changed after the test has started.

Scopes: forms.write

Rate Limiting: This endpoint is rate limited to 400 requests per minute.

{
  "autoClose": {
    "endsAt": "2026-09-30T23:59:59Z",
    "stopOnSignificance": true
  }
}