How to buy a game on PC and XBox One to play cross-platform?

At home I have a PC and a Xbox one. I want to play Deep Rock Galactic on my PC against my wife on the XBox One. I understand they are cross platform. I read I cannot do it from Steam to XBox one. I need to buy both from Microsoft. When I log into the Microsoft
store, It seems to treat my PC account and my XBox one account as the same. What exactly do I need to buy to accomplish this?

:)