In a saucepan heat the milk just until it starts to simmer.
Off heat. Completely cool then remove the mint. If desired, add a few drops of the watermelon flavor. Store in the refrigerator, in a closed plastic container.
Place the juice in the saucepan, add the mint, and cook over medium heat for about 10-15 minutes, occasionally stirring. The amount of the liquid should have reduced. Decrease heat, add the sugar, and simmer for the about 10-15 minutes until the sugar has fully dissolved and the liquid reminds the syrup.
To make the syrup, process the watermelon chunks in the blender and strain from the seeds and flesh.