Booking car ferry to islands without car registration details

Split to Supetar (Brac island) ferry crossing is a first come, first served service, so you can not actually reserve a car place on that ferry  (at least not for the time being)

You can indeed buy your tickets in advance, online here:

shop.jadrolinija.hr/shop/

However, it is just a ticket with no guaranteed place for a car.

So, I suggest you to come early in the port and place your car in the waiting line.

For the Split to Vis car ferry –  yes, you can book and reserve a car place.

To book your tickets now just select vehicle type as ‘other’, enter the dimensions you expect the vehicle to be, and then enter vehicle registration as ‘unknown’.

There is also a “rent-a-car vehicle” box which you should also click – see below screenshot:

You can check all details about booking ferries and car ferries in Croatia at this page: https://www.croatiaferries.com/booking-croatia-ferries/