Apparently there is no such thing as a Ducati 916, 748, 851, or 888 in the Superbike options. I must obviously own a fake, or have been lied to by Ducati for all of these years.
Yeah, I wouldn't bother entering if I were you. And if you have, just delete all your entries, it's not worth it.
So the official answer is that you can obviously still register these bikes, but if it's not in the list then please select "Other" (at the bottom) as the type and then type in the model name, eg "916". They even sent me a little video in case I didn't understand Add Bikes Owned: Ducati > Family: Other > Model: 916 > Year: select year Screenshots below. HTH.