Not familiar with the Cobra or the 21T control. With MDI on my 18T control it would take several lines to do it, so I do not even bother trying. Can you turn it in the jog mode? How about just writing a simple complete program using the info from the manual?
If this was no help try in the CNC part of the forum. I know that some use Cobras there, every now and then someone gets a CNC answer here but manual machines are discussed more often than not in this part of the forum.