For redirect to work:
• HTML form (no JavaScript): redirect is applied automatically; no changes needed.
• Submit with fetch: in your code: (1) in headers set Accept: "application/json" and (2) after const json = await r.json() add if (json.ok && json.redirectUrl) { window.location.href = json.redirectUrl; return; }.
Minimal example (replace YOUR_ENDPOINT with your form URL):

const r = await fetch("YOUR_ENDPOINT", {
  method: "POST",
  headers: { "Accept": "application/json" },
  body: new FormData(form)
});
const json = await r.json();
if (json.ok && json.redirectUrl) { window.location.href = json.redirectUrl; return; }

In Forms, click the code button (icon </>) for your form to see the “Code to integrate” block with your URL (AJAX, React or JavaScript tabs).