Auto Complete Extender ContextKey assignment without $find method

I have a Ajax Auto Complete Extender which works perfectly to get the data as user types, but now I wants to go one step further and filter the resultset. so I am trying to pass the contextkey in javascript, but my $find method doesn't work for some reason. I try to use this generic code that we used all time, but for some reason, it doesn't return anything when I do $find.

 if($find('<%=CompleteExtenderXYZ.ClientID%>')){$find('<%=CompleteExtenderXYZ.ClientID%>').set_contextKey($get("<%=txtPQR.ClientID %>").value);
        }

Any idea what is alternative way to set the Contextkey?? using any other syntax??