In order to control a programmable tailstock, your post processor needs to have the function enabled.
If you have created your own post processor using the Universal Post Generator (UPG), you can enable the Tailstock by going to the Optional tab and checking the box for Tailstock. To look at the code, go to the Sections tab and under Miscellaneous there will be a field for Tailstock In and Tailstock Out. This is where you can modify the codes.
Tailstock moves reference the tailstock home position and not the working envelope of the machine. Tailstock Out therefore refers to the Tailstock engaging with the stock and supporting it. Tailstock In sends the tailstock back to its home position.