Apple soju isn't a traditional drink per se, unless by traditional you mean traditionally found in Korean bars and clubs. I first had apple soju served in a carved out apple in Los Angeles, and have mimicked the recipe below. (Try here for yogurt and other soju recipes). I find a ratio of 2:1 tonic to soju produces just the right amount of alcohol to flavor.
2 apples, washed and carved out
lemon or lime
several ice cubes
1/2 cup soju
1 cup tonic water, chilled
2 tsp frozen apple juice concentrate
1/4 apple, peeled
Cut the top off the apples and carve out with a spoon. Slice the lemon/lime in half and rub it all along the rim and squeeze juice in the inside to prevent the apple from turning brown. It also adds a nice tang to the drink.
Put the ice cubes in the blender and blend until ice is fairly fine. Add in soju, tonic water, apple concentrate and the quarter slice of apple. Blend until well mixed and apple is chopped into small bits. Taste and adjust ingredients if you like.