razorpay refund process flutter

I am using razor pay flutter package to make payment in flutter in test mode . I have options to handle the events and also save the payment id in my database . I want to refund the amount based on the payment id . I dont have much clarification on how to do the same . Here is my code for sending payment request

 Razorpay? _razorpay = Razorpay();
 void openPaymentPortal() async {
var options = {
  'key': 'My_key',
  'amount': 100000,
  'name': 'test',
  'description': 'Payment for Pro account',
  'prefill': {'contact': '9999999999', 'email': 'build@test.com'},
  'external': {
    'wallets': ['paytm']
  }
};
try {
  _razorpay?.open(options);
} catch (e) {
  debugPrint(e.toString());
}
}

void _handlePaymentSuccess(PaymentSuccessResponse response) {
Fluttertoast.showToast(
    msg: "SUCCESS PAYMENT: ${response.paymentId}", timeInSecForIosWeb: 5);
onSubmit(false, response.paymentId.toString());
}

void _handlePaymentError(PaymentFailureResponse response) {
Fluttertoast.showToast(
    msg: "ERROR HERE: ${response.code} - ${response.message}",
    timeInSecForIosWeb: 4);
}

void _handleExternalWallet(ExternalWalletResponse response) {
Fluttertoast.showToast(
    msg: "EXTERNAL_WALLET IS : ${response.walletName}",
    timeInSecForIosWeb: 4);
}

How to process refund of the amount to the user ? I followed something like the below code but it is not working

void processRefund(String paymentId) async {
var url =
    Uri.parse('https://api.razorpay.com/v1/payments/$paymentId/refund');

try {
  var response = await http.post(
    url,
    headers: {
      'Authorization': 'Bearer My_Key',
      'Content-Type': 'application/json'
    },
  );

  if (response.statusCode == 200) {
    // Refund processed successfully
    print('Refund processed successfully');
  } else {
    // Refund failed
    print('Refund failed with status code: ${response.statusCode}');
    print('Response body: ${response.body}');
  }
} catch (e) {
  // Error occurred while making the refund request
  print('Error occurred during refund process: $e');
}

}

I solved my case as follows

    Future<void> processRefund() async {
    String keyId = 'My_Key';
    String keySecret = 'My_Secret';
    String paymentId = 'My_payment_id';

    String url = 'https://api.razorpay.com/v1/payments/$paymentId/refund';

    Map<String, dynamic> refundRequest = {
      'amount': 1000,
      'speed': 'normal',
      'notes': {
        'notes_key_1': 'Tea, Earl Grey, Hot',
        'notes_key_2': 'Tea, Earl Grey... decaf.',
      },
      'receipt': 'Receipt No. #31',
    };

    String basicAuth =
        'Basic ${base64Encode(utf8.encode('$keyId:$keySecret'))}';

    try {
      final response = await http.post(
        Uri.parse(url),
        headers: {
          'Content-Type': 'application/json',
          'Authorization': basicAuth,
        },
        body: jsonEncode(refundRequest),
      );

      if (response.statusCode == 200) {
        // Refund request successful
        var responseData = jsonDecode(response.body);
        print('Refund success: $responseData');
      } else {
        // Refund request failed
        print('Refund request failed with status code ${response.statusCode}');
      }
    } catch (error) {
      // Handle any exceptions
      print('Error creating refund request: $error');
    }
 }