This is from the RouterOS manual. Replace login.html code with this. Edit the URL to yours, of course.
<html>
<title>...</title>
<body>
<form name="redirect" action="https://auth.example.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="$(username)">
<input type="hidden" name="link-login" value="$(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>
Insure to put the url of the server in "/ip hotspot walled-garden" so it will redirect ok.