Here is the actual reverse shifter internals. You will notice that you must put the "box" on the actual shift lever tab but there is not enough room IMHO. Also there are 2 arrows what Ducati calls the Gear Change Control lever. Line the arrow up with the detent on the selector nearly 180 degrees up and your are setup for GP shift. You can always setup GP shift without the reverse quick shift. However I highly recommend turning off DQS if performed.
Also, Ducati was very liberal with the loctite on the bolt securing the shifter to the selector rod and prevents the bolt from moving cleanly out of the ball socket and you may find that the bolt carries the shifter away from the selector rod. If this happens, you have 2 options.
Option 1 No Recommended but might work) Loosen the shifter bolt enough for the lever to be noticably "stuck" and then while lightly heating the area with a blow drier tap the shifter toward the engine to break the bond between the bolts excessive loctite and the ball joint.
Option 2 BETTER)remove the clamping bolt on the selector so that the control lever can move closer to the engine casing and then finish loosening the shifter bolt that threads into the end of the selector rod. If not, you risk bending the shifter and damaging the threads on the selector rod and bolt. Move the selector to the detent 180 up and then screw in the shifter bolt followed by the installation of the control lever clamping bolt. Let me know if you have any questions.
Picture one is the "pull" standard and picture two is the "push" reverse style
Lastly, if you don't have a chapman set or something equivalent, it is quite difficult to get standard hex keys onto the clamp bolt in the standard configuration. See the pic of the chapman set in the yellow box.