I think you can change installation drive location in the xbox app settings not by just including it in the typed path.
But wait a minute, why would you want the program files (about 20GB) on a different drive to your OS and other programs? MSFS actually runs from RAM so performancewise there’s nothing to be gained by moving it from C: However your package files will get accessed while playing so there’s every reason to keep them separate, preferably on a drive of their own with it’s page file disabled (and others increased). My tip, install as normal but when asked for the package location point it to a new folder on the other drive (where hopefully you have aready moved your content if you have any).