The Promotion Code report contains the list of people who used a promotion code to register for the event.
The report can be accessed by clicking "My Accounts" in the left menu, then clicking the "Reports" button for a particular event.
You can run the report, then look in column C to see what promotion code was used.
You can create a custom report by clicking the "Customise" button next to a certain report type.
To learn how to customise a report, click here: https://support.registernow.io/support/solutions/articles/11000069878-creating-customising-reports