changing tdc bearing with the periscope?

[fullmetaledges]

Hey I couldn't figure out how to change the tdc's bearing to target with the periscope, I tried it with the target locked and unlocked, the only way I was able to move the position keeper toward the stern of the ship was to use my hydrophones and send that bearing to the tdc. How to do change the tdc's target bearing using the scope crosshairs?