Prepare the chicken by boiling, grilling, or baking until fully cooked, then shred it into bite-sized pieces.
Chop the celery, red bell pepper, green onions, and parsley into small pieces.
In a large mixing bowl, combine Greek yogurt, mayonnaise, Dijon mustard, apple cider vinegar, garlic powder, onion powder, salt, and black pepper. Mix until smooth.
Add the shredded chicken, diced celery, diced red bell pepper, sliced green onions, and chopped parsley to the bowl with the dressing. Stir gently until coated.
Cover the bowl and refrigerate for at least 30 minutes before serving.