Execute a code at ModalPopupExtender close event execution

Question

I want to call a line of code where modal popup extender closes.But debugger does not comes at this point. is the any other way to do this.. Like:

<asp:LinkButton ID="lnkfaketree" runat="server"></asp:LinkButton>
                <asp:ModalPopupExtender ID="mpeTree" runat="server" BackgroundCssClass="modalBackground recharge" CancelControlID="btntreeclose" PopupControlID="paneltree" TargetControlID="lnkfaketree"></asp:ModalPopupExtender>
        <asp:Panel ID="paneltree" runat="server" Style="display: none">
            <div class="btn-group pull-right">
            </div>
            <div class="box box-primary">
                <div class="box-header with-border" style="padding: 3px; background-color: #14181a; font-weight: bolder">
                    <h3 class="box-title" style="color: white">Tree View</h3>
                    <div class="btn-group pull-right">
                        <asp:LinkButton ID="btntreeclose" runat="server" CssClass="fa fa-close" OnClick="btntreeclose_Click"></asp:LinkButton>
                    </div>
                </div>

                <div class="form-horizontal">
                    <div class="box-body">
                        <div id="chart_div" style="width: 1000px; height: 500px; overflow: auto">
                        </div>

                    </div>
                </div>

            </div>

        </asp:Panel>

And Here is the Line of Code :

protected void btntreeclose_Click(object sender, EventArgs e)
        {
            hdmcode.Value = lblAffiliateCode.Text;
        }