At our shop, they have a consolidated screen with the ids listed. The IMS control regions show as ACTIVE or INACTIVE.
So they have abstracted it for us and all I have to do is put in the eight character name in the menu screen and that program connects my 3270 session to the IMS control region. I honestly wouldn't know how to connect to it any other way.
Chances are, the region to which you want to attach has something similar.