Вы можете выполнить HTTP-запрос POST, используя Apache HTTP Client 4.x :
String xmlString = "... your data ..."
HttpPost httpRequest = new HttpPost("https://localhost/server/api/login");
httpRequest.setEntity(new StringEntity(xmlString));
HttpClient httpclient = new DefaultHttpClient();
HttpResponse httpResponse = httpclient.execute(httpRequest, new BasicHttpContext());
if (httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK && httpResponse.getEntity() != null) {
//handle response ok
} else {
//handle error
}