I genuinely have no idea why you'd want to do this. You do realize that the AppleTV isn't meant to be suddenly cut off from the power supply, right? It's designed to go into standby as its way of being "turned off". What you're doing is the AppleTV equivalent of ripping out a MacBook Pro battery mid-operation, because you're "done using it for the moment."
The simple way of turning your AppleTV off is to hold the Home Button on the AppleTV remote or the Remote app on iOS, and pressing Sleep. OR you can just enable CEC and the AppleTV will auto sleep once you turn off your TV. OR you can just set a short sleep timer on the AppleTV and it'll go to sleep after the time you set.
I get wanting to have voice control over everything, but the way you're doing is it not only inconveniencing yourself, but also a total wasting a HomeKit automation device.