Add it to the top of the page
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
the content part
<asp:Button ID="btn_employee" runat="server" style="display:none" />
<cc1:ModalPopupExtender ID="mpAuthorization" runat="server" PopupControlID="PanelEmployee" TargetControlID="btn_employee"
CancelControlID="Button5" BackgroundCssClass="modalBackground" BehaviorID="behavior1" >
</cc1:ModalPopupExtender>
<asp:Panel ID="PanelEmployee" runat="server" CssClass="modalPopup" Style="display:none; margin-top:50px;" >
<asp:Button ID="Button5" runat="server" Text="X" style="margin-left:480px" ForeColor="White" BackColor="Red" />
</asp:Panel>