...rabbits can be easily trained to use a litter tray, sometimes with more reliability than your average cat!
The natural instinct of a wild rabbit to use one area as its latrine is still apparent in its domestic counterparts. Source - apbc.org (1)
The actual process is very similar to pad training a dog or litter box training a cat. Keep the rabbit confined to a small area while training, move any "accidents" to the litter box, and the rabbit will naturally start using that area for its business. The source link has the details.
(1) Litter Training Your Rabbit
Emma Magnus MSc
Association of Pet Behaviour Counsellors
apbc.org.uk