You may need something to couple stepper motor to threaded rod (like this below, check the diameters - stepper shaft will be 5mm, your threaded rod will be something else)
https://www.ebay.com.au/itm/19-25-mm...WEg:rk:13:pf:0
Or, do it like Aaron did.
BTW, bent threaded rod is OK because tracking speed will not change as the barn door opens.. but straight rod can also be used, the necessary change in speed can be achieved in the program (called "sketch" for whatever reason..)