How to enter voucher code to a subscription

When I signed up for my iFit account it came with a voucher code good for a year. I received another gift card for iFit at Christmas time that is good for 1 year iFit. When you add the voucher code onto your current account it should extend your membership by another year. But ifit says a third party vendor now has my subscription which is Apple. Why can’t I add another voucher code to my membership????

Apple only gives me the option to renew the subscription at some outrageous inflated amount and nowhere to enter a voucher code?? Does anyone have any thoughts on this or what I should do????

Please help!!!!