Are you referring to the Related to dropdown list within the Calls, Meetings, Tasks, and Notes modules? If yes, then you will need to create a custom relationship to Activities. Go to Studio > Custom Module > Relationships > Add Relationships > Select Activities on the "Related Modules" side and click Save and Deploy.
This will add the custom module to the Related To dropdown list within these activity type modules. This will also add the activity type subpanels under the Custom Module's record view.
Yes I was referring to the Related to dropdown list within the Calls, Meetings, Tasks, and Notes modules. Thanks for your instruction, it solved my quandary.
