We recommend downloading the Active For Good app to use your phone as your tracker. It connects to your phone's health data app (iOS: Health; Android: Fit), so you can also enable 3rd party apps to be data sources on your phone's health app & that will then push to your challenge dashboard! See here for instructions on how to do that (iOS/Android).
Active For Good can also connect directly to your Fitbit or Garmin account. For more information, see how to add a tracker here.