GA Tracking before redirection

ga-redirect-tracking.html

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXXXX"></script>
<script>
    window.dataLayer = window.dataLayer || [];
    function gtag(){dataLayer.push(arguments);}
    gtag('js', new Date());

    gtag('config', 'UA-XXXXXXXX');
</script>
<script>
  function redirect(url) {
    gtag('event', 'your_event', {
      'event_callback':
        function () {
          window.location.href = url;
        }
    });
  }
</script>
<script type="text/javascript">
  redirect("https://your-external-url.com");
</script>