In order to be sure that your workout is counted towards the Activity Challenge, you'll need to use any app that adds yoga workouts to Apple's Health app. The first-party Workout app for Apple Watch offers Yoga tracking as well. Once completed, you'll gain the award in the Activity app, under the Limited Edition awards section, as well as a set of stickers that can be used in Messages.
Apple has hosted a few Activity Challenges so far in 2019, including one for Earth Day, one for Heart Month, and one that was exclusive to Apple employees. Each challenge always tasks the user with completing an exercise of some sort in a given time frame, and the rewards typically offer stickers for Messages and a new badge in the Activity app.