The drink served is called the "Watermelon Sugar, Die". Watcher gives the ingredients:
Ice
1.5-2 oz Vodka (depending on how strong you like it)
2 oz chilled watermelon juice
1 oz fresh lime juice (about ½ lime) lime squeezer)
1 teaspoon agave nectar, honey or maple syrup (optional sweetness)
Garnish: Fresh mint, lime wedges, small watermelon wedges
Sparkling water/sprite
Add watermelon juice, vodka, lime juice, and your choice of sweetener syrup to a rocks or cocktail glass then stir to combine.
Fill glass to the top with ice then serve with fresh mint and garnish with a lime wedge or watermelon wedge. Alternatively, you could add the ingredients into a shaker, fill it halfway up with ice, then shake to chill.
If you like your drink fizzy, feel free to top it off with about 2 ounces of club soda or even Sprite.