<!DOCTYPE html>
<html>
<head>
  <script>
  "use strict";

  onload = () => {
    let url = new URL(location);
    if (url.searchParams.get("post")) {
      let server_redirect = `${url.searchParams.get("server_uri")}?redirect_uri=${encodeURIComponent(url.searchParams.get("redirect_uri"))}`;
      let form = document.forms.testform;
      form.setAttribute("action", server_redirect);
      form.submit();
    } else {
      let end = new URL(url.searchParams.get("redirect_uri"));
      end.searchParams.set("access_token", "here ya go");
      location.href = end.href;
    }
  };
  </script>
</head>
<body>
  <form name="testform" action="" method="POST">
  </form>
</body>
</html>
