How to redeem Fitness+ free trial with new Apple Watch

When Fitness+ first came out I signed up for the free 1 month trial that I was offered with my existing Apple Watch 3. I later got an Apple Watch 6 for Christmas. When I activated the new watch I was offered the additional 3 months free trial but at the time I didn't redeem it and swiped it away.

Now that my original 1 month trial is about to end I want to redeem the 3 month trial but I can't figure out how to do so.