<asp:TextBox ID="tbTeamNo" CssClass="text" runat="server" onclick="GetTeamOfBranch();"></asp:TextBox>
                                <asp:AutoCompleteExtender ID="acTeamNo" runat="server" TargetControlID="tbTeamNo"
                                    EnableCaching="false" UseContextKey="true" MinimumPrefixLength="2" ServicePath="../Service/Service.asmx"
                                    ServiceMethod="GetTeam">
                                </asp:AutoCompleteExtender>

function GetTeamOfBranch() {
            var ddBranchNo = document.getElementById("ddBranchNo");
            var acTeamNo = document.getElementById("acTeamNo");
            
            if （acTeamNo != null）
                acTeamNo.set_contextKey("1001");
        }