You can create an appointment for a customer from the admin.
Manual bookings work only for appointment types where payment is not required.
Follow these steps to add appointments on behalf of a customer:
In Shopify, go to Apps → SLOT Appointment Booking → Appointments.
Click Add new appointment.
Choose the product/service and the appointment type.
Note: Only appointments that don’t require payment can be created here.
Select the date and time.
Enter the customer’s name and email.
(Optional) Turn on Send email confirmation to the customer to email the booking details and reminders.
Click Save.