You tried wrong palces and did not read far enough. In the general overview (click on help above) it is utmost clear; ONLY at counter in HRvatska

There are just a few trains, there are many more much quicker buses (pay), but the route seems very populair at the mo, so good luck with it

@Ulrich Albrechtsen  because Croatia didnt load their trains this year in the International booking systems. Because of that its only possible when in Croatia.

What date and time do you want to travel? 

Where did you try to make the reservation?

I´ve tried everywere… DB, ÖBB, DSB, Be, SNCF - also physically sales.

With no luck… Not even in München HBF is able to issue these…. :-( 

And they are mandatory...