I was using the browser simulator for custom SmartApps at: https://graph.api.smartthings.com/ide/apps
The gear shows up after you select a sensor. However, I just tried the simulator a few minutes ago and I can’t even get it to set the location. Not sure if it’s just a temporary bug or if it’s because of a recent firmware upgrade. Either way the simulator doesn’t work for me now.
I really wish they would just let us pick the default display through the mobile app.