How to execute a class

I have a class that is called “World_Gen” (it actually should generate a universe instead of a world but I couldn’t test it yet). It has a beginplay event but it’s just in the content browser, not anywhere on the map.
I would want it to be executed like a piece of code at the beggining of every game but I don’t know how to do it with UnrealEngine yet
What do I do to make it be executed at the start of the game?

Best way would just to drag and drop in you level and if you play your game BeginPlay will fire.