straight from the manual:
<html>
<head>
<title>...</title>
</head>
<body>
<form name="redirect" action="https://my.login.server..com/login.php" method="post">
<input type="hidden"name="mac" value="%mac%">
<input type="hidden" name="ip" value="%ip%">
<input type="hidden" name="user" value="%user%">
<input type="hidden" name="link-login" value="%link-login%">
<input type="hidden" name="link-orig" value="%link-orig%">
<input type="hidden" name="error" value="%error%">
</form>
<script language="JavaScript"> <!-- document.redirect.submit(); //--> </script>
</body>
</html>