score:3
According to the Man in Seat 61 the answer is that you cannot book on-line. Your alternative is to book through one of the agencies he lists or buy at the station while you are in Romania.
https://www.seat61.com/international-trains/trains-from-Bucharest.htm#Bucharest_to_Budapest
Note this is Bucharest to Budapest not from Timisoara but the rules will be the same.