I've been scratching my head over this for a while now, because I believe it's doable, but I just can't get it to work. Here's the situation:
I'm trying to play Elder Scrolls Online through Steam Link, but I've got the game on Epic Store. So I add a non-Steam game to play it, right? Problem is that ESO has its own launcher, so if I open that launcher through Epic, Steam Link "hops" out of Big Picture mode, making Steam Links' controller overlay unresponsive (I believe). And if I open ESO's exe file directly, thus skipping its launcher, I can't login, because my login credentials need to come from Epic, not from a username/password.
An alternative is that I remap the controller overlay to have keyboard inputs, but that would fill up my entire screen with buttons, could take a while to set up, and would be very annoying to play that way.
I hope someone understands my issue and can help me figure this out.
Feel free to ask questions if it helps paint the picture.