The puzzle for Freemarch is in the Lake of solace area As this is an underwater puzzle I recommend you go to Breaker Point at Ember Isle and get 2 hours of underwater breathing from /setwaypoint 11935 3589
The puzzle is at the bottom of Lake of Solace at /setwaypoint 5996 6140
Swim across the water to the waypoint and then swim down to the puzzle area.
The Puzzle
The puzzle is a circle of lanterns that appear and disappear all around you
You need to click each of the lights as they appear til they all turn blue You will need to click them quite quickly before they disappear again, just keep spam clicking them when they pop up The last 2 or 3 lights may take longer to appear as they cycle through, be patient 🙂
Once all the lights are blue the reward chest will appear in the middle of the puzzle.