How to get Reservations | Community (eurail.com) Seewulf has an “tutorial” how to get reservations for Nightjet and in your case an Euronight (Nightjet partner). There you can also look how much it would cost. 

depends on what type of accomodation you want for the night trip. Just a seat is only a few eur, sleeper/couchette cost more-roughly a little more as 1 night in a hostel/hotel-bed.

Note that it seems impossible for THIS train (its Croatian railways stock) to book only the seat/bed online, you have to contact a station, best in germany itself.

Untill München you can use also the fast ICE trains without REServation, but if you fear its busy, it only cost 4 eur and can be done online on bahn.de- again see seewulf and his porting

@mcadv  if you follow the steps of the manual mentioned above you can reserve this Nighttrain via oebb.at online