To use your Apple or iOS device as your Fitness Tracker in KrowdFit you must first download the KrowdFit app from the App Store. Once installed, you will be asked to sign in with your KrowdFit subscription. After you sign if you've not connected a device previously you will be prompted to allow KrowdFit to use your Apple Health information. Once you grant permission you are done.
If you have previously connected a device, you will need to navigate to Settings > Fitness Trackers and add your Apple Health connection from there.