1) In the ecobee mobile app, open the menu from the top left corner (either the green "back" arrow if you're on the main screen of your thermostat or the silhouette icon if you're in My Devices screen)
2) Select "Manage HomeKit" from the menu
3) You should see the option for "Geofence"—select it
4) Select "Enable geofence"
5) Change the radius by pinching the radius-circle to increase or decrease the radius of the geofence
6) You may select a Scene that will trigger (or choose to do nothing) when you enter the geofence boundary and another for when you leave the boundary. Once you've finished making your adjustments, select "Done" in the top right corner to save
7) Success! You've now set up a geofence. Any time you enter or exit the set radius, your specified Scenes will trigger