Any sub domain we provide would work on any server, so you'll need to chose which one you want of the two you supplied.
Also as pointed out by descalzo you should post from the linked forum account for any support requests so we know it is your account you want to make changes to.
If you do not know what forum account is attached to the hosting you can see information, and if you wish change the linked forum account, if you login to the portal and go to "Get Support" -> "Visit Forums". This will provide you with information about currently linked forum account as well as give you an option to switch to another existing forum account.
Once you re-linked, or checked which account you should use, just post back in this thread with the domain you want from the linked forum account.