Sunshine is a host application on your computer that replaces the Nvidia Shield streaming app. Since Nvidia removed that function from the current builds of the Nvidia app you can use Sunshine instead. It also works for Intel and AMD GPUs.

Once you have Sunshine installed and synced with your steam deck you can add any application to Sunshine to allow streaming. In the Xbox PC app find your game executeable location by going to the game page you want to add, then go to More Actions menu>Manage>Files>Browse>Game Folder>Content folder. Add an Application to Sunshine by going to the Applications tab and +Add New. Copy the path of the executeable to the Command field and type in the name of the App. That's the minimum required to run it, but you can also add boxart. Usually it can find it automatically though which is nice.