Passing Class Object to WCF Service c# wp8

Question

I'm trying to pass the class object via Post Async Method

public async void SAveDetails(clsPickList obj)
{
    using (HttpClient client = new HttpClient())
    {
        HttpContent content = new StringContent(JsonConvert.SerializeObject(obj));
        content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
        var result = await  client.PostAsync("http://-Port-/-svc file name-/InsertUserDetails", content) ;
        result.EnsureSuccessStatusCode();
        if (result.Content != null)
        {
            var responseContent = await result.Content.ReadAsStringAsync();
        }
    }
}

to

[OperationContract]
[WebInvoke(Method = "POST",
    ResponseFormat = WebMessageFormat.Json,
    RequestFormat = WebMessageFormat.Json,
    BodyStyle = WebMessageBodyStyle.Wrapped,
    UriTemplate = "InsertUserDetails")]
string InsertUserDetails(Model.clsPickList obj);

and Insert object value as follows

public string InsertUserDetails(Model.clsPickList obj)
{  
    cmd.Parameters.AddWithValue("@UserName", obj.UserName);
    cmd.Parameters.AddWithValue("@Password", obj.Password);
}

but when I Access th Link via WP8 it returns 404 error. Since I'm new to WCF ,I really Appreciate anyones Help.

Thanks in advance..