how to use context key in ajax autocomplete extender using c#

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C#

Ajax

create a web method and need to pass two values one from text box and anather is prefix value

using (SqlConnection conn = new SqlConnection())
        {
            conn.ConnectionString = ConfigurationManager
                    .ConnectionStrings["ConnectionString1"].ConnectionString;
            using (SqlCommand cmd = new SqlCommand())
            {
                //string cmdText = "select ContactName from Customers where " +
                //"ContactName like @SearchText + '%'";

                string cmdText = "select MSupplier.pcode,MSupplier.pdesc from MM_Pomast inner join MSupplier on MSupplier.pcode=MM_Pomast.pcode " +
                " where MSupplier.loccode='" + loccode + "' And  MSupplier.divcode='" + divcode + "' And MSupplier.clicode='" + clicode + "'" +
                " and pdesc like '%'+@prefixText+'%'";

                cmd.Parameters.AddWithValue("@prefixText", prefixText);
                if (contextKey != null)
                {
                    cmdText += " and  MM_Pomast.pordcode = @contextKey";
                    cmd.Parameters.AddWithValue("@contextKey", contextKey);
                }
                cmd.CommandText = cmdText;
                cmd.Connection = conn;
                conn.Open();
                List<string> customers = new List<string>();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    while (sdr.Read())
                    {
                        customers.Add(sdr["pdesc"].ToString());
                    }
                }
                conn.Close();
                return customers;
            }

where i use this
in this prefix value is showing but value of @contextKey is not getting
then i use bellow java script in cs

function SetContextKey() {
        $find('AutoCompleteExtender1').set_contextKey($get("<%=Drppordcode.ClientID %>").value);
    }

set onkeyup for that perticular text box like bellow
onkeyup="SetContextKey()"

but still it not me given an value at context key

need a help very..