Hello

Is it possible to book a reservation seat on another way then the interrail website?

We will take the  night train on September 13th from Ljubljana to Zürich. On the interrail website and the website of OBB (I guess this is the train company), it’s not possible… I’ve checked on other dates and this is the same for all dates, so I guess that the train isn’t fully booked?

Does someone now another way to check it or to make a seat reservation?

A lot of thanks in advance!