This is possible with a little HTML/JavaScript on your Custom HTML signup success page.
<html>
<head>
<meta http-equiv="refresh" content="2; URL=http://site.com">
</head>
<body>
Click here if you are not redirected in 2 seconds: <a href="http://site.com">Site</a>
</body>
</html>
You may also be able to use JavaScript, which would be instantaneous:
<script> <!—
window.location(‘http://site.com’);
--> </script>
