<!DOCTYPE html>
<!--
  There's two URL parameters supported here:

  window: The property name of a getter on the global object that returns the relevant WindowProxy
          object to message.
  ident:  A reasonably unique identifier that will be echoed as the message.
-->
<script>
  const params = new URLSearchParams(location.search);
  self[params.get("window")].postMessage(params.get("ident"), "*");
</script>
