Playing Heroes of the Storm with Steam Overlay

Sort of how you must actually use the direct Diablo 3 executable to launch it as a non-Steam game, so too must you use the direct HotS executable to launch it as a non-Steam game.

The difference is, with HotS it's not as simple as the direct executable instead of the launcher sitting in the same directory. What you are looking to launch is:

PATH/TO/HEROES/Support/HeroesSwitcher.exe

You will need to ensure that the game is kept up to date via the Battle.net launcher, however, and you will need to log in as normal, but as you can see in this screenshot, it works fine.

(This information was found on the official HotS forums and verified by personal testing.)