Remove that field from your layout.
Set up the Customer ID fk field with your value list for selecting a field. This will need to be a value list based on the records in Customers and should list the CustomerID as field 1 and Customer Name as field 2.
And you may find that you have a more basic issue here. Isn't it possible that more than one customer will need to be linked to the same Product?
If so, you'll need a different relationship that uses a join table in order to manage the resulting many to many relationship.
I'd use three tables.
Products >---- ProdCust ----< Customers
Products::ProductID = ProdCust::ProdID_fk
Customers::CustomerID = ProdCust::CustID_fk