keys? are you talking about input events? if thats the case they can actually be called from any bp.
you need a reference to the actor that contains the light so you can target and affect it. there are many ways to get a reference and it depends on the situation as to which method to use. theres traces, public variables, overlap events, etc we have covered this topic an inordinate amount of times on the answerhub.
assuming your topdowncontroller bp is a player controller, and assuming your attempting to do something like a light switch, you could easily get the controlled pawn then get overlapping actors which will return an array then use a find node and search for the object class you need.
you can also use a for each loop as shown below. the script in the example below gets all the actors of light bp class that the character is overlapping then sets the lights intensity contained in the light actor.
