Vouchers are only available on the app through our missions. You need to complete the mission and click the claim button. Once you have clicked the claim button, you should be able to see the voucher on the Rewards page (you may need to refresh the Rewards page for it to show up). They’ll show up as an orange block under your Yeokens balance. You can click on this to see some more information on the voucher.
Remember: Once you have the voucher, you only have 14 days to use it, so don’t hang about!