When clicking on the promotion tab you can see which voucher codes have been consumed by the customer. 

You can also add existing voucher codes manually by clicking on “Add” next to activate a new campaign. Then copy and paste the code into the field and click verify.

The code manipulates the price for a specific subscription for a specific length of billing cycles.

Note: When the time of a discount (e.g. 3 months 50% off) expires the subscription continues and won’t be cancelled automatically. If you apply a second code to an account for the same subscription while the first one is still active it will overwrite the first code immediately.