If you didn’t do so when originally scheduled, you can add a Zoom Meeting to already-scheduled Appointments. Just click to modify an Appointment and, so long as it’s of a type that is set to 'Always' or 'Usually' have a Zoom Meeting you’ll find a button by which to do so:
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When modifying an already-scheduled appointment, you can see the corresponding Zoom Meeting that has been scheduled:
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Note the “Location” field has been set to the exact URL by which you and your clients will be able to access the Meeting.