Like Erasio just said, if you make a variable on Actor B and expose it on spawn, then pass it Actor A as you spawn Actor B, Actor B will spawn with a reference to Actor A in that variable.
But what do you mean by “Actor A will change throughout the level” ?
Do you mean that you want to update Actor B’s reference to whoever Actor A is to a “new” Actor A?
If so it sounds like you should have a variable referencing Actor A in your GameState (or somewhere that is more globally accessible) and then have Actor B look at that variable whenever you need to update it, instead of having the reference on Actor B itself.
On Actor B, create a variable of the type Actor A and make it editable and expose on spawn. This will allow you to change that variable on the ‘spawn actor’ node if you select Actor B as the class to spawn.
On Actor A, spawn Actor B using a SpawnActor node and pass a reference to self (aka Actor A) to the Actor A variable.