Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/external/wpt/html/semantics/forms/form-submission-0/resources/getactionurl-iframe.html 

<!DOCTYPE html>
<body>
<script>
  const form = document.createElement('form');
  document.body.appendChild(form);
  form.action = `data:text/html,
 <!DOCTYPE html>
 <body>
 <script>
   window.top.postMessage('PASS', '*');
 <\/script>
 `;
  form.submit();
</script>