This is an advanced web hook service. We will create a custom table to hold your promo codes. The first step for you is to upload a list of promo codes in this table.
Every time this web hook is called, the next unused promo code will be fetched and will be tagged with the email address you passed. That way, you can tell what lead have used that promocode.
Once all the promo codes are exhausted, this web hook will return 'No promocode found message'
Thus, in your Marketo program, you can make decisions to assign an unused promocode, to notify that the promocodes are exhausted and to be able to report on which all leads used promocodes etc.