New job and how to ask for time off for preplanned appointment

So I start a new job on the 26th. Yay for me but I’m a bit nervous about one thing. Before I even applied for the job, I booked a specialist doctors appointment. I cannot miss or move this appointment. It took years (yes, years) to get. I haven’t gotten my full schedule yet but I’m already worried that I’ll be scheduled for the same day I have the appointment. I’m also scared to start off on the wrong foot by asking for time off on the first few days of the job. Any advice? Should I wait until I get my full schedule to see if I need to ask for time off or should I bring it up right away? Should I try to get in the good books by saying I can go home to the appointment (it’s virtual due to COVID) and then come back to work?

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