well that's done through you hosting cpanel, isn't related to mybb.
regards
Yes, forwarding the subdomain should do it, but as Zaher said, this is a cpanel problem, not MyBB! Contact your host if you are confused!
As far as I know, you can not make subdomains on files. I might be wrong though.
Why don't you just make the subdomains redirect to the real page?
Otherwise, you can make a subdomain for your forum, and a subdomain for the portal via cPanel. Move the portal.php into the portal subdomain htdocs and edit portal.php where it says $forumdir = "./"; to something like "../forum/" (depends on your subdomain setup). Then rename portal.php to index.php.
You could use mod_rewrite if your server is Apache.
You can also use some PHP code like the following:
<?php
$domain = explode('.',sprintf($_SERVER['HTTP_HOST']));
$subdomain = strtolower(str_replace("http://", "", $domain[0]));
if ($subdomain != "www" and $subdomain != $domain[1])
{
switch($subdomain) {
case 'portal':
$url = "portal.php";
break;
case 'index':
$url = "index.php";
break;
case 'whatever':
$url = "whateveryoumean.php";
break;
default:
$url = "index.php";
}
header("location: ".$url);
} else {
header("location: index.php");
}
?>