Once you redeem your subscription voucher code, press the Edit Order page on your subscription home page. Below your subscribed mixes, you'll see a "Have a loyalty code? Enter it here" text box. Copy & paste the code and press apply. It will then successfully apply to your next renewal order.
Note: There are voucher codes that are valid for subscription and one-time orders only. Choosing the appropriate voucher code for your order will ensure a successful application.