Hi,
So, the ListenTo array from ObserveReact is per Actor Instance. That is so you can have one actor blueprint, and multiple actor blueprint instances in your map, and each actor instance listens/reacts to a different list of actors.
Of course, you can manually add items to this array in your blueprint. There are several examples in the Demo Map, where I added items to ListenTo Array. Please take a look at these puzzles:
but, from what I understand from your post, you just want to trigger a button.
So what you could do, is simply call PK_DoAction(NextState) on that button, from your trigger. You can inspect how this Collision Action Triggers are programmed in blueprints.
Let me know if this helps.