Dudas sobre reservas

Hey, ÖBB can’t sell for this train the reservation. It should be possible at ticket counter of MAV Start in Hungary. Maybe it could be also sold by CD ticket office in Prague or ÖBB ticket office in Vienna. 

And yes it is a bit risky, but you could think about to travel to Zagreb with a day train and then from there on a nighttrain to Split (but this is also risky, because, the nighttrain from Zagreb to Split also needs a reservation.

You could ask directly MAV Start the train company that runs this train, how you can buy this reservation from outside Hungary.