The quickest way to claim is online through the app or My Southern Cross. You'll need to choose 'add a medical claim' then enter the name of the event (eg. Round the Bays or Auckland Marathon) in the 'Provider' field; and 'Being Active benefit' in the 'reason' field. The claim must be submitted after you've paid the entry fee and completed the event. You will need to upload a receipt or other proof of entry fee payment, and proof that you completed the event (eg. a certificate from the event organiser).
For information on what is covered by the Being Active benefit, click here.