Add the tajin to a small plate (we like to make a little pile in the center the press the glass on). Line the rim of a glass with lime juice and press into the pile of tajin turning after each part of the glass is coated. Once the rim is formed, fill with ice and set aside.

Add ice to a shaker. Pour over the agave, watermelon juice, tequila, cointreau, and lime juice. Add in 3 jalapenos. Shake for about 30 seconds until very cold.

Strain the drink over the tajin rimmed glass with ice (the ice from the shaker should not make it into the drink glass). Add the jalapenos from the shaker back into the glass if they are strained out with the shaker ice and serve.