Isse with PXSelector

I am new to Acumatica and am struggling to get a selector working in the purchase order screen. I want to add a customer field to the screen in the header area. I have created a custom field and want to use a selector to allow the user to choose the customer for the PO.

Here is the custom field:

        [PXDBString]
        [PXUIField(DisplayName = "Customer")]
        [PXSelector(typeof(SearchFor<Customer.bAccountID>
                             .In<SelectFrom<MOSCustomerProject>
                             .InnerJoin<BAccount>
                             .On<Customer.bAccountID.IsEqual<BAccount.bAccountID>>>),
                    new Type[] {
                        typeof(Customer.bAccountID),
                        typeof(BAccount.acctName)
                    },
                    DescriptionField = typeof(BAccount.acctName),
                    SubstituteKey = typeof(BAccount.bAccountID))]

The issue that I am having is that the selector does not get populated with the customers. If I manually put a customer’s bAccountID in the database field, the correct customer's name is displayed but opening the selector does not show the list of customers.