C# Sending SMS via Huawei 303 ... this is not COM and AT command

using System;
using System.Net.Http;
using System.Windows;
using System.Net;
using System.Text; // for class Encoding
using System.IO; // for StreamReader

namespace WpfSMShilink
{
    public partial class MainWindow : Window
    {
   
        public MainWindow()
        {
            InitializeComponent();
        }
             
        private void SendSMS()
        {
          DateTime Fecha = DateTime.Now;
            var SMS = $"<?xml version='1.0' encoding='UTF-8'?><request><Index>-1</Index><Phones><Phone>";
            SMS += Telefono.Text;
            SMS += "</Phone></Phones><Sca></Sca><Content>";
            SMS += Mensaje.Text;
            SMS += "</Content><Length>";
            SMS += Mensaje.Text.Length;
            SMS += "</Length><Reserved >1</Reserved><Date>";
            SMS += Fecha.ToString("yyyy-MM-dd HH:mm:ss");
            SMS += "</Date></request>";

            var baseAddress = new Uri("http://192.168.8.1");
            using (var handler = new HttpClientHandler { UseCookies = false })
            {
                using (var client = new HttpClient(handler) { BaseAddress = baseAddress })
                {
                    var request = (HttpWebRequest)WebRequest.Create("http://192.168.8.1/api/sms/send-sms");
                    var postData = SMS;
                    var data = Encoding.ASCII.GetBytes(postData);

                    request.Method = "POST";
                    request.KeepAlive = true;
                    request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
                    request.ContentLength = data.Length;
                    request.Accept = "*/*";
                    request.Referer="http://192.168.8.1/html/smsinbox.html";
                    
                    using (var stream = request.GetRequestStream())
                    {
                        stream.Write(data, 0, data.Length);
                    }

                    var response = (HttpWebResponse)request.GetResponse();
                    var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();
                    Info.Content=responseString;
                }
            }
}

        private void BtnSend_Click(object sender, RoutedEventArgs e)
        {
            SendSMS();
        }
    }
}