If a Purchase Order number is available, the associated Customer Invoice can be found by using a query.
To create a saved query, go to Hub / Utilities / Queries. Type the query as seen in the screenshot below.
Choose Save Query and give the query a meaningful name. Select OK. This query will now be available to choose at this screen.
To run the query, go to Hub / Utilities / Queries and choose the query saved above from the Saved Queries drop-down. Replace xxxxxx with the Customer's Purchase Order number. Leave the quotes around the Purchase Order number. Select Show Results to retrieve the Invoice number.
