The question is worded strangely, not sure what you mean by using the blueprint. That really is dependent on your game.
The usual way is to pass the reference of the controller along from the point that the event occurred. If it’s a direct button press then it’s on the controller itself, if it’s a trigger volume in the world then you have the Actor that did it, you can get the Actor’s controller and cast to your type, if you are mouse clicking into the world again you have the controller at the button click. Just pass the controller as a reference.