How can the user manually link Customer to IGT Order?

We have coded logic in the PA_CLIENT_EXTN_IGT_CUST package to derive the customer on the Agreement for IGT orders. However, we are trying to test what happens when more than one customer is derived, so customer is not automatically assigned.

Error message states: The specified customer does not exist. Please select an existing customer or add this customer using the Add Customer form.

Question: how do we select an existing customer from the GINV dashboard? I have not been able to find an option to specify the customer. When we select the order and then select Create Agreement, we get an error related to a savepoint not being established.