A bubble tea (or in this case, coffee) drink consists of roughly these parts:
- a liquid (in this case, coffee or espresso)
- sugar and / or a flavoring ingredient
- tapioca pearls
- ice to dilute and chill it
In your case, the use of sweetened condensed milk make it “Vietnamese coffee”.
If you want to get some additional flavor into the drink, use the coffee shop staple flavor syrup - the same product that is used to flavor other espresso-based drinks. They should be readily available in your local supermarket (Monin being a well-known brand for example), online at the usual retailers, or your nearest coffee chain may be selling their own product. If you are so inclined, a websearch will also lead to lots of home-made recipes, especially for flavors like caramel or vanilla.
So how would a substitution work?
In short, you can either
- use the syrup in addition to the sweetened condensed milk (will be extra sweet as you have two sugar sources)
- use it instead of the sweetened condensed milk (if you are not interested in the milky component)
- use it with unsweetened milk, condensed or regular, or coffee cream for the creamier mouthfeel.
For a large drink, a splash of syrup should be intense enough, although it depends a bit on the chosen brand. Start with a teaspoon or two, add more to your liking.