Place 1/2 cup apple cider vinegar, 1/4 cup ketchup if using, 1 teaspoon hot sauce, 1/2 teaspoon kosher salt, 1/4 teaspoon red pepper flakes, and 1/4 teaspoon black pepper in a jar, liquid measuring cup, or small bowl. Cover and shake if using a jar or whisk until combined and the ketchup dissolves. Let sit at room temperature for at least 4 hours to allow the flavors to meld before serving.