a simple way to do it would be to have the user touch the object on the screen where they want to add the annotation instead of having them select an area.
Then on the touch get the touch coordinates, covert that to 3d coordinates, and pass that to a line trace which can hit the hidden actor. Then get the location from the hit result and use that location to spawn your annotation(s).
Then you’ll need to add a UI widget to have them type in the text. then pass the text to the annotation BP.
-e