Tricopter tail servo

I am trying to build a tricopter using a pixracer and copter 3.5. I have everything set up except I can’t figure out how to set up the tailservo. In mission planner, there is a servo output page under initial setup, which I’m guessing is how i set up the servo. In the function drop down menu for servo #1, I selected “RCIN_4”. I’ve also tried “rudder” and a few other things. I got the green bar that says “position” to move with input from the radio the first time i tried it but now it doesn’t move no matter what i select, with the radio calibrated and working. Not sure what else to do. Thanks in advance to any help!