I started down the subscription path to see if something popped up but for some reason I didn't see anything like I've used in the past. When I've renewed online there's been a place to put in your renewal code from your WM renewal documents.
I would recommend you email subscriptions to confirm the procedure you need to use. They are always very good and helpful when I've contacted them. Helen Slater was an absolute delight to deal with and I would highly recommend checking with her if at all possible at: