I understood you want to run your game on SteamDeck, SteamOS main library.
It works for me and here is what I found out:
what didn't work:
- first I added the "start" script via right klick in desktop mode "Add to Steam"
- this added the game to steam libraray in SteamOS, but it wouldn't start. I guess the directories were set wrong.
- I removed the game from the library
what worked:
- I went to desktop mode and opened Steam.
- there I clicked on "Add non-steam game to steam"
- I added the "start" script (I renamed it before to my games name)
- I started SteamDeck in normal mode again (SteamOS)
- I can now run the game and also use the buttons etc.
Actually, running adventure games on steam is a lot of fun and I am already developing for steam deck controls, using action zones in combination with button actions etc.