Sweetened condensed milk adds sweetness and a thick texture. Put it in the cup below the phin so you can stir it into the coffee after brewing.
For a larger, 8 oz phin, the
Vietnam Tourism recipe collection gives a Vietnamese iced milk-coffee recipe using two tablespoons of sweetened condensed milk. Use that reference with the matching brewer and coffee-and-water quantities, then adjust sweetness to your preference.
If you use fresh milk or a plant-based drink instead, the sweetness, texture and dilution will change. Write it as a separate recipe and taste the finished version rather than swapping equal amounts and expecting the same result.