This probably isn't the spirit, but the simplest thing is to right-click on the map and pick "Add note", that's definitely participating, although there's an "o

This probably isn't the spirit, but the simplest thing is to right-click on the map and pick "Add note", that's definitely participating, although there's an "outsiderness" to it, you have to hope an "insider" (someone who has created an account) actually works on the map to address your note... someone unlike Wikipedia where both anonymous and logged-in users get direct editing access to the underlying data.