I made this because of the lack of guides that do it this way when I looked to do it with my clean install, and didn't really want a whole game installed on my pc that i wasn't planning on using anyway.

First up, add the program/exe you want to have a overlay, In this case it'll be stalker GAMMA/the mod organizer file.

In your steam app, click on add a game in the bottom left.



Click browse.



Navigate to the mod organizer executable in the GAMMA root folder (default is c:\GAMMA)

Next, go to Steamdb and look for the game you want it to count as. In this case I'm using Call of Pripyat.

Look for the Steam ID (41700)



Now, go to the root folder of GAMMA, or wherever the exe is you added as a non-steam game, and create an empty file named steam_appid.txt



Now, open with a txt editor of choice and write the steam id in there.

Now all you have to do is run the game you added through steam



And it will count as the game you used the steamID of.



It's that simple, and this works with any game and any exe added as a non-steam game (I assume...)

Hope y'all enjoyed this, this is primarily a reference for myself in the future 🫡