I did that but it did not work. I had to use crappy iTunes and I had to put my password in about four times but after it still shows Apple Music renewing on Sep 8th 2016 and my account says I have a Apple ID balance of $99. I called up customer service had him remove the number and we did it again on the phone again using the music extension but nothing changed. Still says Sep 8th 2016 with a $99 Apple ID balance. They said to call them back on my renewal date if I get charged the normal monthly rate. A real mess that I could not even do it on a web browser unlike every other service out there.
I had a mare with it as well, 30 minutes on the phone to Apple support. Redeemed the code(UK ebay purchase) It never even showed up on my account. Apple had to remove it then I had to add it back so now it shows as redeemed and is in my account in itunes but doesnt even show in Apple music on iPhone at all. In Apple Music on iPhone my account says I'm subscribed with my monthly renewal coming off next month at 9:99. You'd expect the app to show the renewal as Sept 6th 2017 but nope its showing as October 6th 2016. An absolute F up.