How to enable session in web service web method

[WebMethod(EnableSession = true)]
public CascadingDropDownNameValue[] GetCountryData(string knownCategoryValues, string category)
{
Thread.Sleep(500);
List Countries = new List();
StringDictionary sd = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);

int RegionId = Convert.ToInt32(sd["undefined"]);

CountryByRegionIdTableAdapter cbrta = new CountryByRegionIdTableAdapter();
CountryDataSet.CountryByRegionIdDataTable cbrdt = cbrta.GetCountryByRegionId(RegionId, SessionManager.CaseStudyUserID);

foreach (DataRow dr in cbrdt.Rows)
{
Countries.Add(new CascadingDropDownNameValue(Convert.ToString(dr["CountryName"]), Convert.ToString(dr["CountryID"])));
}

return Countries.ToArray();
}