Once you have generated a set of discount codes, you may want to distribute them to your existing subscribers or as part of your welcome message for new subscribers. In most email platforms, however, the welcome message use case is not possible. That's one reason why we decided to build an email platform ourselves. If you'd like to start including a unique code in your welcome message, please visit seguno.com to learn more and request early access.
As for accomplishing the use case of sending a unique code to existing subscribers with any email platform, such as MailChimp, you can export the discount codes to CSV and then import those codes into your email provider. Each system is different so please reference your email provider's documentation on importing data into a contact field and how to then insert the field into emails.