can't find the property about onclientitemselected on AutoCompleteExtender

Hi,

   why can't I find the property about onclientitemselected on AutoCompleteExtender.This is my code:

<ajaxtoolkit:AutoCompleteExtender ID="AutoCompleteExtender1"
 MinimumPrefixLength="1" TargetControlID="txtAutoCompleteforCompany"
 CompletionSetCount="10"CompletionInterval="100"
 ServiceMethod="GetCompanyList"
 ServicePath="../UiHelper/CompanyServiceForAutoSuggest.asmx"
 runat="server" OnClientItemSelected="setCompanyMasterID"
 CompletionListElementID="listPlacement">

I can't use the property about OnClientItemSelected. Have any idea?

And how can I find the ID of AutoCompleteExtender1 inside in formview1 and masterpage? like this-

function txtAutoCompleteforCompany_onkeydown() {
            var txtAutoCompleteforCompany= document.getElementById('<%=FormView1.FindControl("txtAutoCompleteforCompany").ClientID%>');
            var AutoCompleteExtender1ClientId = '<%=FormView1.FindControl("AutoCompleteExtender1").ClientID %>';
            var AutoCompleteExtender1 = $find(AutoCompleteExtender1ClientId);
            if (AutoCompleteExtender1!= null)
                AutoCompleteExtender1.set_contextKey(txtAutoCompleteforCompany.value);
                    }

Thanks.