If you have a discount or promotional code that you want to add to your current subscription just simply sign in to your mypura.com account and go to Account. Click on My Subscriptions, under your subscription you will see an Add Discount button. If you click this, you will then be able to add the code in the pop up box and select Apply Discount Code to apply this to your subscription.
If the discount or promotional code has a time limit you will receive the discount on charges that are processed while the discount or promotional code is active.