You can reserve directly at oebb.at (Follow the Nighttrain reservation manual in the link below ) You have to pick your reservation from a Austrian Ticket office or Ticket machine :)
 

Thank you so much! :D
I will look into it! 

@Zoé  you could visit a “DB Reisezentrum” or a Austrian Ticket office aswell and reserve there directly just show your pass and they should know how to reserve :)

I could reserve via oebb.at even few sleepers beds are free :)

Thank you, I could get a reservation with the link you send me. 
I just have to print it in Austria and I have gut it

This is not true. Good night train appears to be unavailable on their website directly. It can be purchased through Eurail but you have to have them mail you a paper ticket which is kind of hard when you’re on the move constantly. Your phone number is really bad. I spent over $10 in being on hold on that number. Unlike other numbers a charge to centimeters charges over $.18 a minute and it burns to your Skype credit like crazy.

@gregohb  did you follow the manual?

Nighttrain reservations have a strange booking way on oebb.at as you have to first have to click on the ticket option even if you just want to reserve and then afterwards change the settings to Eurail/Interrail reservation :)

Wien - Brasov can be reserved via oebb.at but have to be picked up at a Austrian Ticket office or Ticket machine