import 'package:flutter_oauth2/flutter_oauth2.dart';

OAuth2 handler = OAuth2(Config(
        authorizationEndpoint: endpointUri,
        grantType: GrantType.PASSWORD,
        clientCredentials: Credentials(clientId, clientSecret),
        userCredentials: Credentials(username, password)));

try {
  Token token = await handler.authenticate();
  // do something with the token
} catch (e) {}