just got hired (yay!) but i might need some days off a few weeks away

so the days off that i was looking to get are already scheduled with me in them (they are like 3 weeks away and they had completed the schedule literally a day before i was hired). i know i could post my shift, but is that the only way it could be approved? (if someone takes my shift)? is the only confident way to get approved is if they havent worked on that schedule yet? does that impact the decision/outcome?