Convert String to XML (WEB API) Android Xamarin

I already figured out how to post data on the web api, but the data is not posting, i know that the codes are right but the conversion of text might be wrong (Correct me if I'm wrong)-

This is my Main Acticity

using Android.App;
using Android.Widget;
using Android.OS;
using System;
using System.Net.Http;
using Newtonsoft.Json;
using System.Text;

namespace PostRequest
{
[Activity(Label = "PostRequest", MainLauncher = true)]
public class MainActivity : Activity
{
Button savebtn;
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        savebtn = FindViewById<Button>(Resource.Id.click_meh);

        savebtn.Click += async delegate
        {
            using (var client = new HttpClient())
            {
                //new post
                var post = new visitorsmodel
                {
                    ID = 1,
                    lname = "Square Pants",
                    fname = "Sponge",
                    mname = "Arenas",
                    school = "BOB",
                    email = "spoongebob@gmail.com",
                    num = "0909090909"
                };
                // create the request content and define Json
                var json = JsonConvert.SerializeObject(post);
                var content = new StringContent(json, Encoding.UTF8, "application/json");

                // send a POST request
                var uri = "http://192.168.1.140:9090/";
                var results = await client.PostAsync(uri, content);

                //Error
                results.EnsureSuccessStatusCode();

                ////Getting results
                //var resultsString = await results.Content.ReadAsStringAsync();
                //var display = JsonConvert.DeserializeObject<info>(resultsString);
            }
        };
    }
}

}

When i click the "CLICK MEH" button, the data will post on the Web API (As i know)

and here is my class model:

namespace PostRequest
{
public class visitorsmodel
{
public int ID { get; set; }
public string lname { get; set; }
public string fname { get; set; }
public string mname { get; set; }
public string school { get; set; }
public string email { get; set; }
public string num { get; set; }

    public override string ToString()
    {
        return string.Format(
            "Post ID: {0}\nlname: {1}\nfname: {2}mname: {3}school: {4}email: {5}num:",
            ID, lname, fname, mname, school, email, num);
    }

}

}

**But there's no changes in the Web API that i create using WPF self host.

This is my WEB API**

why i can't post in this web API (XML)?

Thanks in advance