3 options are possible:
The client book online
As soon as a new client book an appointment online, booxi create a new client file.
You book an appointment
As soon as you book a new appointment from your booxi calendar, booxi create a new client file.
You manually create a new client file
Go to the Client tab,
Click on the Add a new client option,
Provide first name and last name and click on new,
Complete the Client Details section with all the information you have and click on save.