IMHO, we can't do that from server-manager
In server-manager, under Configuration --> Hostnames and addresses:
Click *Add hostname* button.
In *hostname* field enter "forum"
In *Domain* select you domain (mydomain.com)
If you want, add a comment, and click *Next* button. Then click *Add* button.
Of course, you must add the "forum" hostname in your DNS provider if not support * hostnames.
Then in your site aplication (phpBB), configure URL to forum.mydomain.com