The starter templates should already have those primary key fields. Open up Manage | Database | fields and look for fields defined with auto-entered serial numbers.
In the case of the Invoicing starter solution, you have a table named "customers" with a customerID field as the primary key. The trick to linking the two files is to change the 4 Table occurrence boxes in Manage | Database | Relationships to refer to the Contact Management table in the Contact management file.
Not only does that require updating these four TO boxes to refer to an external Filemaker Source--contact manager, but you'll use the field K_ID_Contact in place of Contact ID in the relationships in Invoices. And you'll need to compare definition of the similars field in customers with the definition of the k_similars field in contact manager to see if they are the same or if you'll need to add this field from Customers to the Contact Manager table.