Hi @josef,
As per the pixhawk documentation, the pixhawk doesn’t actually supply power to the servo rail - it’s intended that either something else does, or you can power the pixhawk itself from that rail, or the servo rails can be used for just the signals and you power your devices externally.
Your suggested connection diagram should work, except I’d recommend also connecting the ground wire to the pixhawk ground. Ground is used as an electrical reference point, so if your signal voltage is relative to a different reference to the transistor then it could float and give unexpected/undesired results.
From a brief search, I wasn’t easily able to find out what kind of signal the turnigy switch requires to turn on and off. The hobbyking listing mentions that “The Turnigy switch will complete a circuit when the radio channel passes 50%”, in which case I would assume it should work to give it a DC input (as in the relay toggle pixhawk option you’ve said you’re trying to use).