You mean a seat reservation ?

Use tickets.oebb.at : no booking fee and sometimes cheaper than eurail.com. Delivery by email. Add Eurail/Interrail as a discount and select one-way tickets.

It's often best to avoid eurail.com to book seats because they add fees and send paper reservations while they're available online somewhere else.

General advice :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

For domestic trains in Croatia : 

Great, thanks for your help :)

@thibcabe 

I already have an EU global rail pass, when on obb website, do I need to get a ticket or just a seat reservation? 

Thanks

It's counter-intuitive, but on the ÖBB website you need to add the Interrail/Eurail discount and then choose "One-way tickets and day tickets”. That will then result in a reservation for Interrail/Eurail.