Pretty easy using Json.net. You can get it using the nuget package manager in VS.
var postBody = new
{
friend = new
{
name = "dave",
last ="franz"
},
id = "12345",
login = "mylogin"
};
var postString = Newtonsoft.Json.JsonConvert.SerializeObject(postBody);
using(var wc = new WebClient())
{
wc.Headers.Add("Content-Type", "application/json");
var responseString = wc.UploadString(serviceAddress, "POST", postString);
}