Cook the macaroni in a large pot of salted boiling water according to package directions until al dente. Do not overcook.
Drain the pasta in a colander and rinse thoroughly with cold water to stop the cooking process and cool it down. Set aside to drain well.
While the pasta cooks, prepare the dressing. In a small bowl, whisk together the mayonnaise, Dijon mustard, white wine vinegar, sugar, garlic powder, salt, and pepper until smooth.
In a large bowl, add the cooked and cooled macaroni, chopped chicken, red bell pepper, celery, grated carrot, and green onions.
Pour the dressing over the ingredients in the large bowl.
Gently stir everything together until the pasta, chicken, and vegetables are evenly coated in the dressing.
For best results, cover and refrigerate for at least 30 minutes to allow the flavors to meld. Serve chilled.