I am not at my machine so I am doing this from memory but it should get you into the right neighborhood.
If you are using Mach3, go to config / slave axis and under X-Axis, check off the A button. Also, under config / homing limits, check off reversed.
Go to Ports and Pins, Motor Outputs and enable the A axis and reverse the step and direction and add your step and direction port. There may be some other ones that I've forgotten so a good place to find answers to MAch3 questions is the MAch3 forum. If you are using other software , go to their websites.