[Flutter] How to post/send XML data instead of JSON in rest API Flutter?

I would like to send/post data in Rest API with Flutter. But I cannot find any solution on how to send XML file with Flutter.

I need to send this data in post API
<?xml version="1.0" encoding="UTF-8"?>
<Request StartDate="2020-07-05" EndDate="2020-07-05" ServiceID="0">
</Request>


And my code is given below which I tried with flutter

Future<HttpClientResponse> _send() async {
var builder = new xml.XmlBuilder();
builder.processing('xml', 'version="1.0" encoding="iso-8859-9"');
builder.element('Request', nest: () {

builder.attribute('StartDate', '2020-06-02');
builder.attribute('EndDate', '2020-07-02');
builder.attribute('ServiceID', '0');

});
var bookshelfXml = builder.build();
String _uriMsj = bookshelfXml.toString();
String _uri = "https://my_url";

var _responseOtp = post(_uri, _uriMsj);
print("_responseOtp: $_responseOtp");
}


//**POST XML:**

Future<String> post(String _uri, String _message) async {
HttpClient client = new HttpClient();
HttpClientRequest request = await client.postUrl(Uri.parse(_uri));
request.write(_message);
HttpClientResponse response = await request.close();
StringBuffer _buffer = new StringBuffer();
await for(String a in await response.transform(utf8.decoder)) {
_buffer.write(a);
}
print("_buffer.toString: ${_buffer.toString()}");
return _buffer.toString();
}


And it returns this response

_responseOtp: Instance of 'Future<String>'
I/flutter ( 9755): _buffer.toString: <?xml version="1.0" encoding="UTF-8"
standalone="yes"?>
I/flutter ( 9755): <Response type="service-response">
I/flutter ( 9755): <ResponseCode>1020</ResponseCode>
I/flutter ( 9755): <ResponseStatus>error</ResponseStatus>
I/flutter ( 9755): <ResponseMessage>Requête invalide</ResponseMessage>
I/flutter ( 9755): </Response>

• This data works nicely with the postman

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