Mix marinate sauce in a bowl and stir till well blended. Dry chicken with paper towel. Add chicken and all the sauce into a ziplock bag, marinate for 2 hours in room tempreture or overnight in the fridge.
Preheat oven to 200C (350F). Line baking tray with aluminum foil. Drain chicken with paper towel and place on baking tray. Discard marinate sauce. Bake chicken leg for 40-50 minutes, till juice running clear when poking with knife. Place cooked chicken on a plate and let it cool down.
While baking the chicken, add potato, 1 tablespoon salt and water into a big pot. Heat the pot on medium high heat till boiling, and then turn to low heat. Boil the potato till you could easily poke it through with a fork, in about 20 minutes. Place potato in a plate and let it cool down a bit, then transfer into fridge to chill down, for about 20 minutes.
While the chicken and potato is chilling, prepare the vegetables.
Mix garlic, mayonnaise, mustard, lemon juice, rest of salt and pepper in a small bowl, set aside.
Shredded chicken leg meat and cut into bite size.
Dice chilled potato into bite size.
Mix potato, chicken, onion, celery, egg and the sauce mixture into a big bowl or in a pot, mix well with spatula.
Before serving, chill the potato salad in fridge for 1 hour to overnight, in order to let flavor blends well together. (option)