Primary contractor for clients

Some of our clients are our direct clients, meaning we are the primary contractor for the projects and tasks they require. We handle the job and bill them directly. However, other clients come through our affiliated main contractors. In these cases, we execute the work and invoice the main contractor, who in turn bills the client. Particularly for support tickets, we need information not only about the client but also about who the main contractor is. When creating a Client record, I couldn't find a field for this purpose, so I wanted to consult you. How can we define the main contractor for every task (or for the client) we invoice? Thanks in advance!