This has to be done in adventure mode. You should see a puzzle which is not on the track towards the final level. I personally chose to do this in wordament, and the bonus puzzle was next to level 16. First complete all level up until level 16, and then complete the bonus puzzle. This is not that hard, just takes a short while to get there.
Use this solver to make it easier:https://words-solver.com/wordament-cheat/