I don't believe that you can use server-side includes in your custom HTML, but don't know for sure. One alternative, so that you can use your own scripts, PHP etc, is to set up an automatic redirection from the Custom HTML page to one of yours.
ie:
<html>
<head>
<title>Thanks for subscribing!</title>
<meta http-equiv="refresh" content="2; URL=http://yoursite.com/post-signup.php">
</head>
<body>
<a href="http://yoursite.com/post-signup.php">Click here</a> if you are not redirected in 2 seconds.
</body>
</html>
It might be easier / faster with JavaScript:
<html>
<head>
<title>Thanks for subscribing!</title>
</head>
<body>
<script language="JavaScript"><!--
window.location='http://yoursite.com/post-signup.php';
--> </script>
</body>
</html>
