jQuery Grid Plugin - jqGrid

In my jqgrid I have a list of invoices, adding a new invoice I want that the field customerID assume a value.

This is the sql:

SELECT Customers.CustomerID,
        Invoices.FaID, Invoices.FaCustomerID,
        Invoices.InvoiceNumber,
        Invoices.InvoiceData
      FROM Customers
      LEFT JOIN Invoices ON Customers.CustomerID=Invoices.FaCustomerID
      WHERE
        Customers.CustomerID= '.$_SESSION["ID"]

I want in field Invoices.FaCustomerID the value of  Customers.CustomerID

In my grid Customers.CustomerID and Invoices.FaCustomerID are hidden fields

thanks