Hi Interrail community

My girlfriend and I are going on Interrail for the first time this summer. We will travel by the night train from Zagreb to Split (departure: 22:30 10th of august). We would like to book two beds in a couchette car but that does not seem possible at Interrail.eu? Only seat reservation seems to be available but if we check https://www.hzpp.hr/en it seems like the couchette cars are not sold out. Is it possible to book two beds in a couchette car beforehand somewhere else or is it only possible to book this at the train station in Zagreb?

Thank you in advance for answers!

/Frederik