Yugoslavia dosent exist anymore so you dont have Croatian citizenship but you most likely have citizenship for Socialist Republic of Croatia which was part of SFRJ (Socialist Federal Republic of Yugoslavia) except if your parent’s lived in other part’s of SFRJ. If you show them ex-Yugoslav passport this might help you a bit but not much. Anyway just say you have Croat blood, show them Yugoslav passport and tell them that you want Croatian citizenship.
Croatian’s are quite patriotic so i guess they would be happy to give you citizenship.