Originally Posted by ffay005

You must book a ticket where the first flight leaves from a country that uses the euro as its currency. And you may not purchase any add-ons like baggage or seat reservations during the booking process (you can add these to the booking later on through MMB, though).

When these criteria are met, there will be a voucher option on the same page where you fill in your credit card number. Select voucher and type in the requested codes. If the voucher doesn't cover your entire ticket, you can then pay the rest with a credit card.