To add a bank account to a customer we will go to:

Customers -> Customers to access the customer's file.

Click on the "edit" button to access the client's file on the list:

Once inside the customer's file, click on "Additional Information" on the left menu and then on "Bank Accounts" tab.

To add the account press the button "New". (If you want to change the bank account you can press edit and change the number)