On the scheduling page clients will choose a specific time for the appointment. If your business operates by scheduling clients into morning/afternoon/evening slots, or a range within a certain timeframe this is how you'll workaround to get to your end goal. This workaround is great for service technicians or delivery based businesses or any other businesses that require a window instead of a specific start time.
The key here is: (1) limiting the slots available to only the start time for the window of the appointment and (2) communicating throughout the scheduler to the client regarding expectations on the timing of the appointment.
(1) In your Availability enter only the beginning time of the time window. If you have a morning/afternoon/evening slot your availability may be entered as 8am, 12pm, 5pm (depending on when your time windows started). For more on setting up available hours check out this article.
(2) Communicating to your clients on "when" their appointment will take place: