In Inspire, go to Setup > Setup Service > Setup Service
Click + New
Enter a Service Name
Click checkbox for Add On Service,
Note: the service duration fields will be greyed out and un-editable.
Complete the remainder of the checkboxes and fields
Click Save
Add On Services are 0 duration services are only available as Finishing Services for Online Booking. When scheduled, they will not appear on the calendar but will appear in the Appointment List underneath the Appointment Book.