Guided Control for skid steering through companion computer

Hello. Currently I am trying to manually control my skid-steering robot via drone kit through a raspberry pi. I have set up throttle left and throttle right as channel 1 and channel 3 and want to be able control the throttle output via dronekit/mavlink. I have currently been unable to determine the correct message to send to control the two individual motor settings. Any help would be great. Thank you!