Hydrostatic rover

I am building a rover that uses the hydrostatic transmissions out of a zero turn mower. I have a pixhawk flashed with rover but not sire how to hook the pixhawk to use only the right stick on my transmitter. I have a spektrum dx8 set to use the right stick with elevon. That’s where I get confused with making the pixhawk output match.

I’am not sure i understand You but in essential Your radio has at least 4 channels.
Two of them for the left stick x and y axis and another two for the right stick y and x axis.
To help You there is a lot more needed.
Are You using pwm or sbus or something else from Your receiver to fc?
If it is PWM than You have to connect channels that Your right stick is using to the flight controller channels described in Rover Wiki and dependent on Your fc.