Availability starting at certain times only

How can I set up my appointment availability so that they can only start at certain times? i.e. right now I have them set for an hour with 30min buffer but I want to only take appointments starting at the beginning of my availability in 90min increments. eg.) 10:00, 11:30, 13:00, 14:30. So 10:30 or 11:00 should not be available. Thank you.

@yukon You can adjust the start times here:

If you switch this to 90 minutes then the start time will be every 90 minutes.