You can offer a discount to subscribers using Email signup popup type. When you choose this option, you'll be able to add your discount code in either the body text, the 'confirmation' text area (this will show when the user enters their email and clicks 'submit'), or you can add the discount in the email that will come from the mailing list directly.
The discount code must be created separately in the admin of your Shopify or Bigcommerce store.