Example of a custom sanity input that allows the user to select from a list of values retrieved from an external API

ExternalDataInput.js

import React from 'react'
import SearchableSelect from 'part:@sanity/components/selects/searchable'
import FormField from 'part:@sanity/components/formfields/default'
import {PatchEvent, set, unset} from 'part:@sanity/form-builder/patch-event'

const STREET_NAMES = [
  'Thorvald Meyers gate',
  'Champs-Élysées',
  'Lombard Street',
  'Abbey Road',
  'Fifth Avenue',
  'Santa Monica Boulevard',
  'Ginza',
  'Beale Street',
  'Bourbon Street'
]

const delay = ms => new Promise(resolve => setTimeout(resolve, ms))

const searchExternalAPI = keyword =>
  delay(100).then(() =>
    STREET_NAMES.filter(name => name.toLowerCase().startsWith(keyword.toLowerCase()))
  )

export default class ExternalDataInput extends React.Component {
  state = {
    hits: [],
    isFetching: false,
    inputValue: null
  }
  _lastQuery = null
  inputRef = React.createRef()

  handleFocus = () => {
    if (this._lastQuery) {
      this.search(this._lastQuery)
      this.setState({inputValue: this._lastQuery})
    }
    if (this.props.onFocus) {
      this.props.onFocus()
    }
  }

  handleBlur = () => {
    this.setState({inputValue: null})
    if (this.props.onBlur) {
      this.props.onBlur()
    }
  }

  handleChange = hit => {
    const {onChange} = this.props
    onChange(PatchEvent.from(set(hit)))
    this._lastQuery = null
    this.setState({inputValue: null})
  }

  handleClear = () => {
    this.props.onChange(PatchEvent.from(unset()))
  }

  handleSearch = query => {
    this.search(query)
  }

  handleOpen = () => {
    this.search('')
  }

  search = query => {
    this._lastQuery = query
    this.setState({inputValue: query, isFetching: true})
    searchExternalAPI(query).then(hits => {
      if (this._lastQuery === query) {
        this.setState({hits, isFetching: false})
      }
    })
  }

  focus() {
    if (this.inputRef) {
      this.inputRef.current.focus()
    }
  }

  render() {
    const {type, value, level, markers, readOnly, onBlur, onSearch} = this.props

    const {hits, inputValue, isFetching} = this.state

    const validation = markers.filter(marker => marker.type === 'validation')
    const errors = validation.filter(marker => marker.level === 'error')

    return (
      <FormField markers={markers} label={type.title} level={level} description={type.description}>
        <SearchableSelect
          placeholder="Type to search…"
          title={inputValue}
          customValidity={errors.length > 0 ? errors[0].item.message : ''}
          onOpen={this.handleOpen}
          onFocus={this.handleFocus}
          onBlur={this.handleBlur}
          onSearch={this.handleSearch}
          onChange={this.handleChange}
          onClear={this.handleClear}
          openItemElement={this.renderOpenItemElement}
          value={value}
          inputValue={inputValue === null ? value : inputValue}
          renderItem={this.renderHit}
          isLoading={isFetching}
          items={hits}
          ref={this.inputRef}
          readOnly={readOnly}
        />
      </FormField>
    )
  }
}

Thank you so much for making this gist public. I have iterated on it and create a version using typescript and hooks.
https://gist.github.com/ButsAndCats/2cc45dde23c5ba194c9c6038a3fca82e