Prepare a clay cantarito cup by soaking it in cold water for 10 minutes. Rim the cantarito cup with Tajín seasoning. Add the tequila, fruit juices, and salt into a shaker with ice and shake until well-chilled. Strain into the prepared cantarito cup (or a Collins glass) over fresh ice. Top with the grapefruit soda and stir briefly and gently to combine. Garnish with a lime wedge.