Additional Servo + Channel to flightcontroller

Hey, I am trying to connect a additional servo which only should use the pitch, but I want it make it reliable on the position of the flight controller, Like when the flight controller will climb the servo should pitch downwards, when the flight controller is pitching downwards I want the servo to have a positive pitch value. How can I set it up correctly? I found out that I maybe cant use the AUX pins and give it a RC Channel like in example CHANNEL8 but I have to find a way to make the servo move with the flight controller so that I easily then can REV CHANNEL8 is it’s needed then. Can anyone help me with the setup?