Country/Region Selector Input Field for Sanity

Good news! There's actually a plugin specifically for this use case: sanity-country-state-select. This plugin provides exactly what you're looking for - a custom input field for countries and their corresponding states/regions following the ISO 3166 standard.

You can install it via npm:

npm install sanity-plugin-country-state-select

Then add it to your sanity.config.ts (or .js):

import {countryStateListPlugin} from 'sanity-country-state-select'

export default defineConfig({
  // ...
  plugins: [countryStateListPlugin()],
})

And use it in your schema:

import {defineType, defineField} from 'sanity'

defineType({
  fields: [
    defineField({
      title: 'Countries',
      name: 'countries',
      type: 'countrieslist', // the custom type from the plugin
      options: {
        showStates: true, // enables the state/region selector
        placeholder: 'Select country',
        showIcon: true,
      },
    }),
  ],
})

The plugin gives you a nice dropdown interface for selecting countries, and when showStates is enabled, it'll show a corresponding dropdown for states/regions based on the selected country. The data follows the ISO 3166 standard, so you're getting proper country and region codes.

Since you're still getting started with Sanity and React, using existing plugins like this is definitely the way to go - they're well-tested and save you a ton of time. You can browse more plugins at the Sanity plugin library to see what else is available for common UI patterns. Once you're more comfortable with the platform, you can always build custom input components if you need something more specific to your use case.