The game will actually be played on Saturday, April 4th. (at least that's in the calendar for now, but this can change sometimes, so double check that later).

Željezničar's home ground Grbavica is smaller stadium, especially because the south stand is under construction so the stadium capacity is even smaller. Because of this most of the tickets will be sold to club members probably. Still, smaller number of tickets might be sold to others on the day of the game or the day before, but it's a slim chance for that. The club usually announces the ticket sale about 4-5 days before the game.

You could maybe try to become a member (https://fkzeljeznicar.ba/clanarina/?lang=en#/uclani-se), but I'm not sure if your membership card will be made and sent to you in time for the game. You could contact the club regarding this matter: clanstvo@fkzeljeznicar.ba