too long

I have a subdomain setup in cPanel, like example.domain.com, and redirecting to domain.com/blah/file.

This works but of course the URL changes to the destination one and does not stick on the subdomain. It won't navigate anywhere as it is pointing to a holding / campaign page but not sure how I can get it to stay on subdomain name but pull in the URL I want it to.

Is there a WordPress addon to help with this, so it can be managed when creating these campaigns in the admin, or a way in cPanel?

Disable the redirection, point the subdomain to the main document root and edit the .htaccess in your main directory root to include:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^sub.domain.com
RewriteRule ^(.*)$ http://domain.com/subdomains/sub/$1 [L,NC,QSA]

Credit to @Ansari's answer.