Bring a large pot of water to a boil. Add 1 tablespoon of the salt and pour in the macaroni. Cook to al dente according to the package directions. Drain.
In a large bowl, combine the onion, vinegar, and garlic powder. Add the cooked macaroni and toss to combine. Let sit for about 10 minutes.
Meanwhile, in a serving bowl, whisk together the mayonnaise, milk, sugar, tōgarashi, the remaining 1 teaspoon salt, and the pepper until smooth. Add the macaroni mixture and toss to coat in the dressing. Add the scallions, carrot, and celery and toss to combine. Cover and refrigerate for at least 2 hours before serving. If desired, stir in some pineapple and garnish with scallions