To prepare the margarita glass, pour the salt into a shallow plate or bowl. Run a lime wedge on the rim of the glass and dip the rim into the coarse salt. Press to coat evenly.
Pour the tequila, lime juice, sparkling water, and Erythritol into a shaker or mason jar. Fill with ice and shake well until well combined and chilled.
Strain the drink into the prepared glass (optionally over crushed ice) and serve cold, garnished with a lime wedge.