chromium/chrome/test/data/extensions/api_test/browser_action/popup_with_form/popup.html

<html>
<head>
  <script src="popup.js"></script>
</head>
<body>
  <div>This popup contains a frame that triggers a POST navigation.</div>
  <form id="form" method="POST" action="other_page.html">
      Test field:
      <input name="test-field" type="text" value="test-value">
      <input type="submit" value="Submit">
  </form>
</body>
</html>