Steam Deck

Only real way is to install windows. Even then though it's not great. You have to use some third party apps to get windows store to recognize the steam deck controls as a controller. It's a pain in the ass to get working at all and in my experience it's super flakey and prone to crashes. (if you've ever tried to get the steam controller working with game pass games you probably already have a good idea of the problems you might run into).

A slightly more reliable method though is creating a kb+m desktop config for each game you plan on playing through gamepass and manually switching to it before launching your game directly through the windows store. Unfortunately this isn't going to work super great with games that require analog inputs like racing games but it works great for things like fps games or traditional kb+m games.

Steam input is so flexible though that you might be able to figure out a way to get this working right with forza. I'm just not sure how you could. Maybe set throttle in game to some sort of mouse movement and in turn set right trigger to that same mouse movement in your desktop config. That's pure speculation on my part though. I've never personally tried doing that myself. No idea if its actually even possible.