Ducati Hypermotard Owner's Manual page 160

While holding the rear end of the seat (2), insert the
central fixing point by pushing on it.
Push the rear end of the seat (2) until you hear the
bolt in the lock click into place.
Make sure that the seat is firmly secured to the frame
and remove the key (1) from the lock.
Fig 106
2
1
Fig 107
159