How to pass additional parameter to AutoCompleteExtender using dropdownlist?

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ASP.NET4

In my application i m using AutoCompleteExtender to display city list.I want to pass parameter using dropdown. Dropdown contains list of state name and their id. i want to pass id of state from that list of city populated in textbox.pleae help me to solve my problems.

<asp:TextBox ID="txtAutoComplete" runat="server"></asp:TextBox>
        <ajax:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" TargetControlID="txtAutoComplete"
             MinimumPrefixLength="1" EnableCaching="true" CompletionInterval="100"
            ServiceMethod="GetSuggessions"  UseContextKey="true"  ContextKey="<%# Eval(ddlState.SelectedValue) %>"
            CompletionSetCount="20" >
        </ajax:AutoCompleteExtender>

[System.Web.Script.Services.ScriptMethod()]
     [System.Web.Services.WebMethod]

     public static List<string> GetSuggessions(string prefixText, int count, long contextKey)
     {
         // prefixText – text entered in the textbox
         //fill list with suggestive words that will be populated in the list below the textbox


        // List<string> res = new List<string>() { "a", "a1", "a2", "a3", "b", "b1", "b2", "b3" };

          List<string> res = MasterDTO.GetCities(contextKey).Select(s => s.CityName.ToLower()).ToList();
         List<string> suggestions = res
       .Where(a => a.StartsWith(prefixText.ToLower()))
       .Take(count)
       .ToList();
         // contextKey can be used to set further filter
         return suggestions;
     }