How do you activate a custom event in the behavior tree?

Usually the “target” node when activating a custom event between blueprints can be attached to a variable of that blueprint with a tooltipID, but I can’t seem to find a tooltipID for blueprints I have set up in the AI tree. I have been scripting in UE4 for a month but just read about AI trees and it seems like it can make life a lot easier. Any help would be appreciated.