How to set context key in JQuery autocomplete

$('#StateName').autocomplete({

    source: '/Home/GetStateNameSuggestion?CountryId=' + $('#CountryNameHF').val() + '&',
    select: function (event, ui) {
      searchState(ui.item.label);
      return true;
    },
    change: function (event, ui) {
      if (!ui.item) {
        this.value = '';
      }
    }
});

in the above code $('#CountryNameHF').val() is always null.