Status update of a PC


Ok so i am using a Particle photon which will allow me to turn on and off my PC. It works well on the breadboard so far. However what i want to be able to do is to know if the PC is on or not. For example if my kids turn the PC on while i am out. Now I can detect if the PC is on by using a Digital IO on the photon but what i need to understand is how i can send an update to an ST Smartapp from the Photon that just updates the status of the device (off to on and vice versa)
I’ve had a look through the documentation for smartapps and i just about understand using HTTP PUT but what i don’t get is how to force the update. The docs just talk about activating the switch not how to change its status. I have probably missed something simple but just in case.
Any advice or hints would be much appreciated.