const handler = async (event) => {

  let preflightResp = await fetch(
    "https://httpbin.org/response-headers?Flags=group-A,new-header,search-enabled",
    { backend: "origin_0" }
  );

  // Avoid making changes to the original Request object.
  // It's usually better to clone and create a new object.
  const originalReq = event.request;
  const newReq = new Request(originalReq);
  newReq.headers.set('flags', preflightResp.headers.get('flags'));

  const clientResponse = await fetch(newReq, { backend: "origin_1" });
  return clientResponse;
};

addEventListener('fetch', event => event.respondWith(handler(event)));