How to keep undefined value in autocomplete?

I want to keep other search words that are not specified in the Autocomplete.

I am using Autocomplete sample code of flutter. https://api.flutter.dev/flutter/material/Autocomplete-class.html

import 'package:flutter/material.dart';

void main() => runApp(const AutocompleteExampleApp());

class AutocompleteExampleApp extends StatelessWidget {
  const AutocompleteExampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Autocomplete Basic'),
        ),
        body: const Center(
          child: AutocompleteBasicExample(),
        ),
      ),
    );
  }
}

words defined in Autocomplete

class AutocompleteBasicExample extends StatelessWidget {
  const AutocompleteBasicExample({super.key});


  static const List<String> _kOptions = <String>[
    'aardvark',
    'bobcat',
    'chameleon',
  ];

  @override
  Widget build(BuildContext context) {
    return Autocomplete<String>(
      optionsBuilder: (TextEditingValue textEditingValue) {
        if (textEditingValue.text == '') {
          return const Iterable<String>.empty();
        }
        return _kOptions.where((String option) {
          return option.contains(textEditingValue.text.toLowerCase());
        });
      },
      onSelected: (String selection) {
        debugPrint('You just selected $selection');
      },
    );
  }
}