No. You have to apply online, pay the fee, then attend the appointment. https://www.gov.uk/government/publications/usa-apply-for-a-uk-visa/apply-for-a-uk-visa-in-the-usa Appointment slots only show in the online system after you’ve submitted the application. Once booked, it is possible to change an appointment.
It’s also possible to cancel an application and get the fee refunded, provided you do so before the appointment date.