Strava GPX Format for Watts and Power

Strava documentation here is vague. The tags you need to set to extend the Garmin GPX format to allow Cadence and Power look like this:

      <Cadence>0</Cadence>
      <Extensions>
       <TPX xmlns="http://www.garmin.com/xmlschemas/ActivityExtension/v2">
        <Speed>2.5</Speed>
        <Watts>0</Watts>
       </TPX>
      ...

So to add the Watts to a Trackpoint you need to use the TPX tag and set the namespace to an extension and put the Watts in there.