file_postMessageSender.html

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<!DOCTYPE HTML>

<html>

<head>

  <meta http-equiv="content-type" content="text/html; charset=utf-8">

  <title>Test page for always sending a message to its opener through postMessage</title>

</head>

<body>

  <script>

  window.onload = () => {

    // The target origin is coming from the query string.

    let targetOrigin = window.location.search.substring(1);

    window.opener.postMessage("Message", targetOrigin);

};

  </script>

</body>

</html>