You can use the Offers API to add discount codes to an existing offer.

Create an offer

Navigate to the Offers page.
Click + Create offer in the top-right corner.
On the Choose an offer type window, select Unique discount codes
Enter a unique name for your offer in the Discount name field.
Choose the Distribution window.
Note: If you include start and end dates in your API call, the distribution window automatically updates to Custom time period.
Click Create.
Click back into the offer and retrieve the numeric ID at the end of the URL string. This is the ID you will need for the POST