chromium/third_party/blink/web_tests/external/wpt/fetch/private-network-access/resources/xhr-sender.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>XHR Sender</title>
<script>
  window.addEventListener("message", function (event) {
    let { url, method } = event.data;
    if (!method) {
      method = "GET";
    }

    const xhr = new XMLHttpRequest;

    xhr.addEventListener("load", (evt) => {
      const message = {
        loaded: true,
        status: xhr.status,
        body: xhr.responseText,
      };
      parent.postMessage(message, "*");
    });

    xhr.addEventListener("error", (evt) => {
      const message = {
        loaded: false,
        status: xhr.status,
      };
      parent.postMessage(message, "*");
    });

    xhr.open(method, url);
    xhr.send();
  });
</script>