How to do HttpPost request via PostAsync to .NET webservice?

Hi,
I have the error as attached when I'm trying to do a HttpPost request via PostAsync to a .NET webservice? It is just a simple POST request to update a recordset in the database via a passed parameter [id].

As a note, if I'd changed this request to HttpGet, the update worked as expected. This is to validate that there is no issue on the existing webservice.

==========================================
The behind code that triggered by click event (Xamarin)
==========================================
       public async void ClosedPO(object sender, EventArgs e)
        {
            var payload = new Dictionary<string, int>
            {
              {"id", 83832}
            };

            string strPayload = JsonConvert.SerializeObject(payload);
           HttpContent c = new StringContent(strPayload, Encoding.UTF8, "application/json");

            Uri uri = new Uri("https://www.mywebsite.com/webservice1.asmx/PreorderClose");
            var client = new HttpClient();
            var response = await client.PostAsync(uri, c);

            if (response.IsSuccessStatusCode)
            {
                await DisplayAlert("SUCCESS!!", "", "OK");
            }
            else
            {
                await DisplayAlert("FAIL", response.ToString(), "OK");
            }
        }

=========================
The code at webservice1.asmx
=========================
    <WebMethod()>
    Public Function PreorderClose(id As String) As Boolean
        If objProduct.changeProductStatus("preorderclosed", id) Then
            'Successfully updated
            '#Uncomment for production
            Return True
        Else
            Return False
        End If

    End Function

=========================
Web.config at our server
=========================
     <webServices>
        <protocols>
           <add name="HttpPost"/>
           <add name="HttpGet"/>
        </protocols>
     </webServices>

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Error observed from my actual mobile phone when the button is clicked:
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