Popup form using Ajax Model Pop Up Extender in Ap.net C#

<asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
            <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnShowPopup"
                PopupControlID="pnlpopup" CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
            </asp:ModalPopupExtender>
            <asp:Panel ID="pnlpopup" runat="server" BackColor="White" Height="300px" Width="400px"
                Style="display: none">
                <table width="100%" style="border: Solid 3px #D55500; width: 100%; height: 100%"
                    cellpadding="0" cellspacing="0">
                    <tr style="background-color: #D55500">
                        <td colspan="2" style="height: 10%; color: White; font-weight: bold; font-size: larger"
                            align="center">
                            Pay EMI
                        </td>
                    </tr>
                    <tr>
                        <td align="right" style="width: 45%">
                            EMI ID:
                        </td>
                        <td>
                            <asp:Label ID="lblID" runat="server"></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td align="right" style="width: 45%">
                            Buyer Name:
                        </td>
                        <td>
                            <asp:Label ID="lblbuyername" runat="server"></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td align="right">
                            EMI No:
                        </td>
                        <td>
                            <asp:Label ID="lblemino" runat="server"></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td align="right">
                            EMI Amount:
                        </td>
                        <td>
                            <asp:Label ID="lblamount" runat="server"></asp:Label>
                        </td>
                    </tr>
                    <tr>
                        <td align="right">
                            Payment Date:
                        </td>
                        <td>
                            <asp:TextBox ID="txtpaiddate" runat="server" />
                        </td>
                    </tr>
                    <tr>
                        <td>
                        </td>
                        <td>
                            <asp:Button ID="btnUpdate" CommandName="Update" runat="server" Text="Pay" OnClick="btnUpdate_Click" />
                            <asp:Button ID="btnCancel" runat="server" Text="Cancel" />
                        </td>
                    </tr>
                </table>
            </asp:Panel>


in code behind button event on which pop has to be displayed

protected void LinkButton3_Click(object sender, EventArgs e)
    {
        LinkButton lnkbtn = sender as LinkButton;
        GridViewRow gvrow = lnkbtn.NamingContainer as GridViewRow;               
        this.ModalPopupExtender1.Show();      
    }