Am I right that you only need a normal servo output but you are using something like PPM coming out of the receiver?
Make sure your transmitter is configured to send that knob or pot out, a lot of transmitters now days need configuration for that to work. Then go to Configuration -> Input with flight battery plugged in and transmitter on. Wiggle the knob and you should see it in the Receiver Activity in the upper right. Get that working before continuing. Now map that channel to something like Accessory0 and give it reasonable min, neutral, max (1000, 1500, 2000 for PPM). Press Save.
Go to the Vehicle page and map that Accessory0 to an unused "RC Output" in the lower right of the page. Press Save.
Go to the Output page and give that output reasonable min, neutral, max (1000, 1500, 2000).
Now you have that channel appearing on that output on the FC.