How do I add coins automatically to player and show on screen after answering quiz made from widgets correctly
You would just need to setup an event/function that fires off when the quiz is completed. To display it on screen, the “coins” would have to be a text variable in a widget or some sort, you would also need an integer value stored on a player controller/character or game instance (depending on setup) that would keep track of the coins collected, then you write a function to increase or decrease this value depending on what happens in the game and update the text variable of the widget accordingly.
This tutorial series goes over how to do this and a lot more basic stuff you may be interested. Video #10 is what you are looking to do now. I show you how to implement a scoring system displayed on a widget. So this should either be exactly what you need or something very close you can modify. Hope it helps.