Mix cucumber, carrots, and 1 teaspoon of salt for about 10 minutes. Rinse and drain well.
In a medium bowl, add cucumber, carrots, shallots, and Thai Chilies. Pour in hot water, white vinegar, sugar, and ¼ teaspoon of salt. Stir to ensure everything is evenly coated. Adjust seasoning if needed.
Allow to cool to room temperature and transfer to an airtight glass jar, put in the fridge for at least 4 hours before serving. This Acar will stay fresh up to a week in the refrigerator.