Its in the database matey, under 'catalog_vouchers' Its pretty easy to figure out what each column means, but I've included a screenshot
Voucher - The code you want users to enter into the in-game catalog.
Type - Credits OR Duckets, this is the currency users will receive once redeemed.
Value - The amount of credits/duckets you want the voucher to give.
current_uses - This will auto-update in correspondence to the use of the voucher.
max_uses - If this is 0, the voucher will have unlimited uses, however if you want to limit this to 10, this box should read 10.
enabled - 0 = False (Voucher can not be used)
1 = True (Voucher can be used)
Once you have set this, you can run the command ':update vouchers' and your voucher will now be active.
In your database you will see a table called "catalog_vouchers". In there you can add a new voucher for credits/duckets. After adding a new record say :update vouchers and you should be good to go.