The obvious questions:
Have you created forums.mydomain.com using the control panel for hosting a new domain? (domains–>manage–>create new (on bottom)–>create subdomain (forums.mydomain.com)
Did you also use the control panel for the redirect? (domains–>manage–>create new (on bottom)–>redirected–>mydomain.com/forums to forums.mydomain.com
It does take a while for the subdomain to function (usually not too long–minutes).
It seems you want to remap a sub folder to your subdomain.
Try DH panel --> Domains --> Remap Sub-Dir
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