Why forward to an external address? It should be sufficient to add a DNS entry? You are trying to use the firewall the other way around… (or I am misreading your post)
If you want to forward a website the index.html at your local server could be as the following:
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<meta http-equiv="refresh" content="0; url=https://MyExternalSite.com">
<script type="text/javascript">
window.location.href = "https://MyExternalSite"
</script>
<title>Page Redirection</title>
</head>
<body>
<!-- Note: don't tell people to `click` the link, just tell them that it is a link. -->
If you are not redirected automatically, follow this <a href='https://MyExternalSite'>link to https://MyExternalSite</a>.
</body>
</html>