Does your Polar watch have the ability to record a GPS track? If so, you should be able to record a GPS track for the trail you would like to add, download it from your watch, and then upload it to Trail Run Project.
If your watch doesn't have this functionality, another option (if you have an iPhone) would be to use the Trail Run Project mobile app to record the trail. If you don't have an iPhone, but have another smart phone, you could download the Strava app and use that to record a GPS track for the trail.
Hope this helps, and we look forward to seeing your trail!