Should work at a SBB ticket counter.

Can also be done online on tickets.oebb.at. You'll have to book your bike separately from couchette or sleeper, since the bike spaces are in the IC and couchettes/sleepers in the NJ. Also, don’t complete one booking at a time, because you first want to know if there's availability for both. So first add both bookings to your basket and only then complete them.

I have tried many times both the options you suggest (SBB and OBB) and none have worked, they keep sending me from one to the other. The last information I got was that it’s NS who treats the bike spaces (it appears the IC is theirs)…

Sorry, I made a mistake. You can book the bicycle on bahn.com. Again, don't complete the booking until you know can also book a couchette/sleeper/seat via ÖBB.

On bahn.com, you need to replace the passenger with a bike (or multiple bikes). Also, book a ticket for the complete journey you will make that day (if possible), not just for the night train, so you don't need to buy additional bike tickets.

I got them!!!! Thank you soooooo much!!!!