Navigation panel > Relationships > Cards
Search for and select the customer card
click on tab Contacts
Add or select the contact
Make sure the contact is set as follows:
They must have a Full Name (first and last name)
They must have an Email Address
They must have a Phone Number
They must have a Job Description
They must have a Role
They must have either Accounts Payable or Purchasing Officer ticked (flagged)
All other fields are optional and can be filled in if you have those pieces of information.