Try this: set up IdleMaster as a non-Steam game in your Library. Then, open IdleMaster up from your desktop or wherever you have it installed. After that, go back to Steam and launch it as a game. It should open a seperate tab from the original one you had open.

From there, just close the original tab of IdleMaster, the one you didn't launch from Steam, and it should say that you're playing IdleMaster. Check out my profile, I'm doing it right now actually :)