I am not quite sure If I understand your question and actually I have monetized my mobile game differently now so I don’t have any of those Blueprints left. Anyway: There is no event for when the player closes your game. That wouldn’t make sense. Once the timer starts, you need to set the current time to a variable which you can get by the node “Now” and save it. Every second you compare the value from “Now” with the time you have set in the variable and give the correct amount of coins. When the player leaves the game and reopens it, load your variable and simply continue to compare. Then you will now know how much time the player waited.
But as stated in my old post this can be cheated by manually setting the time of the device forward. I guess you have to live with that.