sign_up_screen.dart

Future<void> _signUpUser() async {
  if (_formKey.currentState!.validate()) {
    await Amplify.Auth.signUp(
      username: _usernameController.text,
      password: _passwordController.text,
      options: CognitoSignUpOptions(
        userAttributes: <CognitoUserAttributeKey, String>{
          CognitoUserAttributeKey.email: _emailController.text,
        },
      ),
    );
  }
}