Reservations are not possible online for this train. Either you book a regular ticket online through CFR (your screenshot) and save a pass day or you wait until getting to Bucharest train station to book a reservation. It may be possible to buy that reservation at a DB train station in Germany but unlikely.

Btw there are no seats on this train, only one through couchette carriage attached to various regional trains along the way. An adventure in itself :)

DB cant sell it this year :/ 
Bestwise at the first Romanian Ticket office :) 

It may also be possible to (pre-)book it via email to CFR international :

https://bileteinternationale.cfrcalatori.ro/en/contact

If you try that, do let us know how that works out.

I wil do that. I keep you guys updated.

Thanks for the tip!

I received answer from CFR:

“In your case, tickets can only be purchased from the station. If you buy online, you will pay the full price.”

So my best bet is to buy a ticket when arriving in Romania and hopping that there is still enough seats available.