In that case you should find the master actor from your building. There is no way to listen to a delegate without subscribing to it, and for that you need to know the object that will broadcast it.
I would do it something like this:
void InteractableActor::RegisterBuilding()
{
for(TActorIterator<AYourMasterActorClass> Itr(()); Itr; ++Itr)
{
AYourMasterActorClass* MasterActor = *Itr;
if(MasterActor != nullptr)
{
//Create a function that can be called by the buildings being created
MasterActor->RegisterBuilding(this);
return; //end the execution of the function since we only needed this.
}
}
//Probably shoot out an error if the master actor was not found when attempting to register.
}