How to make a Dropdown String Field Required

{
  title: "Type of post",
  description: "Pick the format of your post",
  name: "type",
  type: "string",
  options: {
    list: [
      { title: "Article", value: "article" },
      { title: "Video", value: "video" },
      { title: "Podcast", value: "podcast" },
    ],
  },
  validation: (Rule) => Rule.required(),
}