Adding a preview of a singleton document on Sanity Studio using sanity-plugin-iframe-pane.

import {ListItemBuilder} from 'sanity/desk'
import defineStructure from '../utils/defineStructure'
import Iframe from 'sanity-plugin-iframe-pane'
import {SanityDocument} from 'sanity'

const isDev = window.location.origin === '<http://localhost:3333>';
const liveUrl = 'localUrl'
const liveUrl = 'liveUrl'

// Customise this function to show the correct URL based on the current document
function getPreviewUrl(doc: SanityDocument) {
  if (isDev) {
    return `${localUrl}`
  } else {
    return `${liveUrl}`
  }
}

export default defineStructure<ListItemBuilder>((S) =>
  S.listItem()
    .title("Home")
    .schemaType('home')
    .child(
      S.document()
        .schemaType("home")
        .documentId("home")
        .views([
          S.view.form(),
          S.view
            .component(Iframe)
            .options({
              url: (doc: SanityDocument) => getPreviewUrl(doc),
            })
            .title('Preview')
        ])
  )
)

preview: {
  select: {
    title: "title",
  }
}