You can register the time you are free before or after your appointment and the time you are free at that time as an appointment.

Edit Booking Page > Set Reception Time > Available time before and after the appointment.

If you register a free time before or after an appointment as an appointment, the time will be registered as an appointment.

Booking page with settings: Candidate dates/times are displayed after the schedule you have set (e.g., travel time).
Other appointment pages: Candidate dates and times are displayed by allocating time before and after the appointment page from the registered appointments (travel time, etc.).