1<script type="text/javascript">
2   // set the target on the form to point to a hidden iframe
3   // some browsers need the target set via JavaScript, no idea why...
4   document.getElementById('my-form').target = 'my-response-iframe';
5   // detect when the iframe reloads
6   var iframe = document.getElementById('my-response-iframe');
7   if (iframe) {
8     iframe.onload = function () {
9       // now you can do stuff, such as displaying a message or redirecting to a new page.
10     }
11   }
12</script>

1<form id="my-form" target="my-response-iframe" action="https://docs.google.com/forms/u/1/d/e/<YOUR-ID>/formResponse" method="post">
2  <input type="text" name="entry.12345678" value="" required>
3  <input type="submit">
4</form>

1<iframe id="my-response-iframe" name="my-response-iframe"></iframe>