Slow Cooker Alabama Pulled BBQ Chicken

Name: Slow Cooker Alabama Pulled BBQ Chicken
Ingredients: 1 onion, cut into 1/4-inch thick rings; 0.75 teaspoon salt; 0.5 teaspoon ground black pepper; 0.5 teaspoon paprika; 0.25 teaspoon poultry seasoning; 3 cloves garlic; 1 (3 pound) whole chicken, skin removed; 1 tablespoon melted butter, or as needed; 0.5 cup water; 0.5 teaspoon liquid smoke flavoring; 0.75 cup mayonnaise; 2 tablespoons cider vinegar; 2 teaspoons white sugar; 0.5 teaspoon salt; 0.25 teaspoon ground black pepper; 0.25 teaspoon cayenne pepper