If you want the user to specify the salesperson to allow them to select any salesperson in the file, then you'd use:
TRIM({Salesperson})=TRIM(QUERYSTRING("Which saleperson?",""))
or if all your salespeople have uppercase codes then:
TRIM({Salesperson})=TRIM(UPPERCASE(QUERYSTRING("Which saleperson?","")))
For an even better user interface, use the Salesperson table in either Invoices or OrderEntry and create a View listing the salesperson code and name. The
Link that table by the salesperson code to the Customer View. Then the user can just click on the salesperson and see their customers.