{
    "tag":"login"
    "email":"email@email.com"
    "password":"P@ssw0rd"
}

HttpClient client = new HttpClient();
        client.BaseAddress = new Uri("http://localhost/login/");
        string link = "http://localhost/login/";

        UserCredentials cred = new UserCredentials(email.Text, pass.Password.ToString());
        var data = new Dictionary<string, List<UserCredentials>>();
        string json = JsonConvert.SerializeObject(data, Formatting.Indented);


        HttpResponseMessage re = await client.PostAsync(link, new StringContent(json));

public class UserCredentials
{
    public UserCredentials(string user, string pass)
    {
        User = user;
        Pass = pass;
        Tag = "login";
    }

    internal string User;
    internal string Pass;
    internal string Tag;
}

<?php

if (isset($_POST['tag']) && $_POST['tag'] != '') {
  echo "got tags";
else
  echo "didn't get anything";
?>

public class UserCredentials
{
    public UserCredentials(string user, string pass)
    {
        User = user;
        Pass = pass;
        Tag = "login";
    }

    public string User;
    public string Pass;
    public string Tag;
}

UserCredentials cred = new UserCredentials(email.Text, pass.Password.ToString());
string json = JsonConvert.SerializeObject(cred, Formatting.Indented);
System.Diagnostics.Debug.WriteLine(json);

{
  "User": "me@hotmail.com",
  "Pass": "ILoveToEatGrapes",
  "Tag": "login"
}

// Create the list of keys and values.
var data = new Dictionary<string, string>();
data["tag"] = json;

// Send my keys and values.
HttpClient client = new HttpClient();
string link = "http://localhost/login/";
HttpResponseMessage re = await client.PostAsync(link, new FormUrlEncodedContent(data));

POST /login/ HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Host: localhost
Content-Length: 142
Expect: 100-continue
Connection: Keep-Alive

tag=%7B%0D%0A++%22User%22%3A+%22me%40hotmail.com%22%2C%0D%0A++%22Pass%22%3A+%22I
LoveToEatGrapes%22%2C%0D%0A++%22Tag%22%3A+%22login%22%0D%0A%7D

$cred = json_decode($_POST['tag']);

UserCredentials cred = new UserCredentials(email.Text, pass.Password.ToString());
    string json = JsonConvert.SerializeObject(cred, Formatting.Indented);

UserCredentials cred = new UserCredentials(email.Text, pass.Password.ToString());
var data = new Dictionary<string, List<UserCredentials>>();
data["key"] = new List<UserCredentials>{cred};
string json = JsonConvert.SerializeObject(data, Formatting.Indented);