Not possible currently. Like many other Microsoft games those are programmed as Universal Windows Platform apps, so they run in a sandbox and can’t be hooked into without serious hacking.
Supposedly Forza Horizon 4 will come to Steam in the not too distant future, with luck that might mean it gets converted to a normal Windows application. Don’t hold your breath, but at least there is some hope.
I have a WINDOWS version of Forza Horizon4
However, I am thinking of purchasing it knowing that the STEAM version will be released.
And I was thinking of trying to make it three-dimensional.
I think it would be great if FORZA HORIZON 4 could be played in 3D.
I hope this game can be three-dimensionalized.