To add a customer in WHMCS, follow the below steps:
-Log in to the WHMCS administration area.
-Go to the “Clients” section.
-Look for “Add New Client” or something similar.
-Fill in the client information (name, email, and password).
-Provide more information, such as contact information, if wish too.
-Give the consumer a login and password.
-If relevant, choose a client group.
-Save or submit the form.
-Send a confirmation email if wish too.
-Confirm the successful addition of the customer.
All detailed are explained at https://help.whmcs.com/m/managing/l/682009-manually-adding-a-client