After he's finished congratulating you, explain that you have decided that it's almost certain, based on your priors, that we live in a simulation, and the being who runs this simulation is God. You call Him Fred. Most likely He's the super(universe) equivalent of a grad student.

I've always thought of a god as something that among other traits, is reasonably high in the hierarchy. An outside-universe grad student would not count, for the same reason that Dracula doesn't.

Furthermore, when most people say that God created the universe, by "the universe" they mean "everything except God himself". A simulation would not count as a "universe" even though we inherently have no way to perceive things outside it.