Sorry I should said it more generally, you cannot set object reference inside BP editor/setup because the engine don’t know what objects are in scenes that this actor will be placed in.
You can only do so after you place it in the scene or change reference type to class/soft reference. But for your purpose you have to place both actors in scene and after that you will find it somewhere in details (in this image case Default > Some Actor Ref and in your Default > Variable For Exit)
I hope this is the solution you was looking for.
