How can I pass a string as parameter? using System.Net.Http;

I am using this line

client.PostAsync("mysite.net/login.asmx/login", content);

And I need that my content be something like id=21&name=myname&password=pass&json=1 It should be a string but the accepted type here is a FormUrlEncodedContent and because of that I don't know how this class create the value, if it works like a json or not and if it will change this structure id=21&name=myname&password=pass&json=1 that I need (it should be the rest of the url) How can I send it and receive the response converting it to my class user, for example?

I am using it as well

var content = new FormUrlEncodedContent(new[]
                    {
                        new KeyValuePair<string, string>("id", "125"),
                        new KeyValuePair<string, string>("mailadress", login),
                        new KeyValuePair<string, string>("password", pass),
                        new KeyValuePair<string, string>("json", "1"),
                     });

How my webservice will receive it? how this class(above) organize the key-value?

my class

class User
{
    public string codigo { get; set; }
    public string nome { get; set; }
    public string email { get; set; }
    public string senha { get; set; }
    public string imagem { get; set; }
    public DateTime dataDeNasc { get; set;}
    public string cidade { get; set; }
    public string estado { get; set; }
    public string telefone { get; set; }
    public string sexo { get; set; }
}

Thank you very much. I really need this help