Do you live together? If so, then you could plan some sort of proposal surprise in-game that she runs into, and then that would CUE your real life proposal. Like taking her to some really romantic place in-game and using the "kneel" emote and then opening a trade window with a diamond ring in it.

Then when she turns to you and asks what you're doing, you're ready on one knee with an actual ring or something.

But if you don't live together and play from separate places, then proposing in-game is a really, REALLY bad idea. Just don't do it.