Most of the time, adding the vehicle registration will auto load the model of vehicle for you.
You can complete a quote without the registration however. You will need to manually select the model of the vehicle and year manually, in the drop down section.
You cannot insurer a vehicle without a registration, but can obviously provide a quote before hand.
Sometimes very new vehicles might fail to show automatically when the vehicle registration is added. On these occasions you can manually search for the vehicle.