Click the "Customers" tab, and the "Add New" button.
Fields with a "*" are mandatory. If you're in a rush, you can fill the space with a period.
The "Payer" field is important.
Payer=True designates them as paying customer. If Payer=False, they will not appear in the "Customers" area of the load screen, and they will be considered a Shipper or Consignee.