According to viagogo.com, if you have a voucher code for Viagogo, you can redeem it following the steps below:
Select the event and tickets you want to purchase and click the Buy button. Please note that the event should be valid for the voucher. In some cases, vouchers are issued only for specific events.
Go to the Review section and click on the Gift Voucher link.
Enter the voucher code in the box.
Click the Redeem Voucher button.
The voucher amount will be automatically deducted from the price.
Please note that it is better to login to your Account before searching for the tickets, because some vouchers can only be used for certain users.