That isn’t the purpose of a SmartApp though, you don’t have “buttons” to interact with in a SmartApp. Liken it to a light switch, the button’s you see to turn it on and off or defined on the device type of the light switch.
You would have to go to the “Thing” to turn it on or off. If you open up the SmartApp, you’ll notice you just see a bunch of settings and configuration options. Not anything that actually directly interacts with the device.
My guess is you’re just confused a bit and I really don’t know which device type you are using for your device, but probably the default SmartThings ones. If you use the one that @Mr_Lucky mentioned, written by @jpansarasa you can turn it on/off from within the “Thing” area. Look here: Aeon Labs Smart Energy Switch DSC06