var request = $.ajax({
        type: "POST",
        url: url,
        processData: false,
        contentType: "application/json",
        dataType: "json",
        data: params,
        error: callbackError,
        success: function(data) {
           alert(data);
        }
    });

public string test(string BirthDay, string login)
       {
            return "успех";
       }

"{"BirthDay":"01.01.2000","login":"test"}"

  var request = $.ajax({
        type: "POST",
        url: url,
        processData: false,
        contentType: "application/json",
        dataType: "json",
        data: params,
        error: callbackError,
        success: function(data) {
           alert(data);
        }
    });

    /// <summary>
    /// Summary description for HRService
    /// </summary>
   [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
   // [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    [System.Web.Script.Services.ScriptService]
    public class HRService : System.Web.Services.WebService
    {

[WebMethod]
       public string ADD(Object param)
       {
            return "param.ToString()";
       }...

{"Message":"Invalid web service call, missing value for parameter: 'param'.","StackTrace":"   at System.Web.Script.Services.WebServiceMethodData.CallMethod(Object target, IDictionary`2 parameters)\r\n   at System.Web.Script.Services.WebServiceMethodData.CallMethodFromRawParams(Object target, IDictionary`2 parameters)\r\n   at System.Web.Script.Services.RestHandler.InvokeMethod(HttpContext context, WebServiceMethodData methodData, IDictionary`2 rawParams)\r\n   at System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)","ExceptionType":"System.InvalidOperationException"}