This very same subject (just scroll down and read) has been posted and/or mentioned here many times. The final conclusion we are still awaiting, but as for now there is NO way to book online the ´free´ (note that you would still have to pay port-fuel-hiseason-bed taxes, together with a passday to use this may mean a normal ticket is cheaper!) trip.

Juniors and seniors can claim a discount of 30/20%. There are also various other ferries-also from Brindisi or Ancona, that may offer even lower end-fares, they nearly all give discounts to ju/se-nior. Just use google and you will find a dozen or so general ferry-booking-sites. Do note however that the last years some ferries only take peoples with cars. Also every year there are reshuffles between the many ferries that run IT-GR and due to covid many run reduced schedules. Superfast now joins with ANEK ferry. IF you want to tour the Greek isles-then there is a Greek Island ferry pass-the most expensive is for 6 trips and 1 of these may include the IT-GR ferry (much longer and thus much more expensive as inland GR).

In the distant past (but that is very long ago) this ´free´ passage was only available as stand by (dus bad luck if boat was full-not unlikely in hi-season) in the port office on the last hours before sailing off. I surfed the websites yesterday and found that for other ferries with IR-discount (to Morocco) this is still the case!

I suppose you're seeing the error below? It's always easier if you specify what "not able to choose” means and mention any errors or where you get stuck.

I'd contact their customer service (helpdesk@anek-superfast.com), mention the error you're seeing and ask them how you can book the free deck passage with Interrail, as mentioned on their information page.