google forms submit form redirect to new thankyou page

foo.html

<script type="text/javascript">var submitted=false;</script>
  <iframe name="hidden_iframe" id="hidden_iframe" style="display:none;"
  onload="if(submitted) {window.location='whateveryourredirectis.html';}"></iframe>
    <form action="YOUR-EMBEDDED-GOOGLE-SPREADSHEET-LINK" method="post" target="hidden_iframe" 
    onsubmit="submitted=true;">