The required anti-forgery form field “__RequestVerificationToken” is not present in ajax call

I have the following method in controller

[HttpPost]
    [Authorize(Roles ="Klient")]
    [ValidateAntiForgeryToken]
    public ActionResult GetAvaiableHouses(DateTime startDate, DateTime endDate)
    {
        Session.Remove("Reservation");
        IEnumerable <SelectListItem> avaiableHouses = repository.GetAllNamesAvaiableHouses(repository.GetAvaiableHousesInTerm(startDate, endDate));

        List<string> houses = new List<string>();
        avaiableHouses.ToList().ForEach(item => houses.Add(item.Value));

        if(avaiableHouses.ToList().Count == 0)
        {
            return new EmptyResult();
        }

        Session["Reservation"] = new NewReservation()
        {
            StartDate = startDate,
            EndDate = endDate,
            AvaiableHouses = avaiableHouses
        };

        return PartialView("~/Views/Shared/_AvaiableHousesPartial.cshtml", houses);
        }

This method is called by using ajax script in View.cshtml

$(function () {$("#btnCheckAvaiableHouses").click(function () {$.ajax({
            type: "POST",
            url: "/ClientReservations/GetAvaiableHouses",
            data: '{startDate: "' + $("#startdate").val() + '",endDate:"' + $("#enddate").val() + '",__RequestVerificationToken:"' + $('input[name=__RequestVerificationToken]').val() +'"  }',
            contentType: "application/json; charset=utf-8",
            dataType: "text",
            success: function (response) {$('#avaiableHouses').html(response)
                if (!$('#avaiableHouses').is(':empty')) {
                    document.getElementById("btnConfirmTerm").style.visibility = 'visible';
                }
                else {$('#avaiableHouses').html('Brak dostępnych domków w podanym terminie')
                }

            },
            failure: function (response) {
                alert(response.responseText);
            },
            error: function (response) {
                alert(response.responseText);
            }
        });
    });
});

This is the section of buttons with this one which call this script

<div class="form-group"><div class="col-md-offset-2 col-md-10"><input type="button" id="btnCheckAvaiableHouses" value="Sprawdź dostępność domków" class="btn btn-default" /><input type="button" id="btnConfirmTerm" value="Potwierdź termin" onclick="location.href='@Url.Action("Create", "ClientReservations")'"  class="btn btn-default" style="visibility:hidden" /></div></div>

I have added the additional parameter

'",__RequestVerificationToken:"' + $('input[name=__RequestVerificationToken]').val()

into ajax script but during execution I still receive the error

,__RequestVerificationToken is not present. 

What could be the reason?