To initialize the Amplify API category you call `Amplify.addPlugin()` method. To complete initialization call `Amplify.configure()`.

Your code should look like this:

```dart
import 'package:amplify_flutter/amplify_flutter.dart';
import 'package:amplify_api/amplify_api.dart';
import 'package:amplify_auth_cognito/amplify_auth_cognito.dart';
import 'package:flutter/material.dart';

import 'amplifyconfiguration.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  void initState() {
    super.initState();
    _configureAmplify();
  }

  Future<void> _configureAmplify() async {
    // Add the following line to add API plugin to your app.
    // Auth plugin needed for IAM authorization mode, which is default for REST API.
    final auth = AmplifyAuthCognito();
    final api = AmplifyAPI();
    await Amplify.addPlugins([api, auth]);

    try {
      await Amplify.configure(amplifyconfig);
    } on AmplifyAlreadyConfiguredException {
      safePrint(
          'Tried to reconfigure Amplify; this can occur when your app restarts on Android.');
    }
  }

  Future<void> onTestApi() async {
    // Edit this function with next steps.
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Padding(
          padding: const EdgeInsets.all(20.0),
          child: ElevatedButton(
            onPressed: onTestApi,
            child: const Text('Rest API'),
          ),
        ),
      ),
    );
  }
}
```