Use any filed available in the Customer section - e.g. below - Job title was used for store codes- Cin7 filed used was Job Title - one per customer card
Then enter the filed name in the Order Exchange Accounting system setting screen
usually all one word lower case
can find field name on the web link below
use the text format shown in the Sample file as the guide for what to enter in the Customer Store Code Field (the format in the table higher up on the page may be inaccurate)