When a Customer Message has been made as the Preferred Customer Message by doing the steps below, that message will automatically default when a transaction is created. However, this is only applicable if the transaction is a stand-alone transaction and not created from another transaction.
1. Navigate to Setup > Accounting > Accounting Lists
2. Set Type = Customer Message
3. Edit the Customer Message you want to make as preferred
4. Mark the 'Preferred' checkbox
If a transaction is created from another transaction (e.g. creating an Invoice/Cash Sale by billing the sales order or creating a return from an Invoice/Cash Sale/Sales Order), the Customer Message that will default or auto-populate on the new transaction will be the Customer Message entered on the 'Created From' transaction at the time the new transaction is created.