This is a known Indonesian drink that you can find almost every where, from street vendor to fancy restaurant.
Step 1: Ingredients
You will need:
Red syrups of any flavors you like, but usually Indonesians use either Ros or Cocopandan flavor (shown on picture). Available online and/or at Asian stores.
Cold club soda
Sweetened condensed milk (not evaporated milk).
Pour as much sweetened condensed milk into serving cup as you like (more for creamier and sweeter), follows with the syrup, the measurement is depending on how much milk you added earlier.
Add in some ice cubes, and just before serving, pour in cold club soda.
Stir and enjoy~
NOTE: This is the recipe that I know since little kid again, however, a month ago I was invited to a party and the host was serving this and yet she couldn't find red syrup, so she used strawberry milk + club soda. It was almost similar to the taste of Happy Soda!