The simplest option for GraphQL requests is to use the `headers` property of a `GraphQLRequest`.

```dart
Future<void> queryWithCustomHeaders() async {
  final operation = Amplify.API.query<String>(
    request: GraphQLRequest(
      document: graphQLDocumentString,
      headers: {'customHeader': 'someValue'},
    ),
  );
  final response = await operation.response;
  final data = response.data;
  safePrint('data: $data');
}
```

Another option is to use the `baseHttpClient` property of the API plugin which can customize headers or otherwise alter HTTP functionality for all HTTP calls.

```dart
// First create a custom HTTP client implementation to extend HTTP functionality.
class MyHttpRequestInterceptor extends AWSBaseHttpClient {
  @override
  Future<AWSBaseHttpRequest> transformRequest(
    AWSBaseHttpRequest request,
  ) async {
    request.headers.putIfAbsent('customHeader', () => 'someValue');
    return request;
  }
}

// Then you can pass an instance of this client to `baseHttpClient` when you configure Amplify.
await Amplify.addPlugins([
  AmplifyAPI(baseHttpClient: MyHttpRequestInterceptor()),
]);
```