how to validate login screen using ajax modal pop extender via onokscript() method?

<tr>
             <td colspan="2" style="height: 80px; text-align: center; background-color: #BDBDBD;">
                 <h1>WebSite Header
                     <asp:LinkButton ID="LinkButton1" runat="server">Login</asp:LinkButton>
                     <asp:ModalPopupExtender ID="LinkButton1_ModalPopupExtender"  BackgroundCssClass="modalBg"
                         runat="server"
                         Enabled="True"
                         TargetControlID="LinkButton1" PopupControlID="Panel1" CancelControlID="Cancelbtn"
                          OkControlID="LoginButton"   ></asp:ModalPopupExtender>
                 </h1>
             </td>
             <td>
                 <asp:Panel ID="Panel1" runat="server"  CssClass="modalPanel" Style="display: none">
                     <table style="border-collapse: collapse;">
                         <tr>
                             <td>
                                 <table>
                                     <tr>
                                         <td style="text-align: center;" colspan="2">Log In</td>
                                     </tr>
                                     <tr>
                                         <td style="text-align: right">
                                             <asp:Label ID="UserNameLabel" runat="server" AssociatedControlID="UserName">User Name:</asp:Label>
                                         </td>
                                         <td>
                                             <asp:TextBox ID="UserName" runat="server"></asp:TextBox>
                                             <asp:RequiredFieldValidator ID="UserNameRequired" runat="server" ControlToValidate="UserName" ErrorMessage="User Name is required." ToolTip="User Name is required." ValidationGroup="Login1">*</asp:RequiredFieldValidator>
                                         </td>
                                     </tr>
                                     <tr>
                                         <td style="text-align: right">
                                             <asp:Label ID="PasswordLabel" runat="server" AssociatedControlID="Password">Password:</asp:Label>
                                         </td>
                                         <td>
                                             <asp:TextBox ID="Password" runat="server" TextMode="Password"></asp:TextBox>
                                             <asp:RequiredFieldValidator ID="PasswordRequired" runat="server" ControlToValidate="Password" ErrorMessage="Password is required." ToolTip="Password is required." ValidationGroup="Login1">*</asp:RequiredFieldValidator>
                                         </td>
                                     </tr>
                                     <tr>
                                         <td colspan="2">
                                             <asp:CheckBox ID="RememberMe" runat="server" Text="Remember me next time." />
                                         </td>
                                     </tr>
                                     <tr>
                                         <td colspan="2" style="color: Red; text-align: center">
                                             <asp:Literal ID="FailureText" runat="server" EnableViewState="False"></asp:Literal>
                                         </td>
                                     </tr>
                                     <tr>
                                         <td style="text-align: right" colspan="2">
                                             <asp:Button ID="LoginButton" runat="server" Text="Log In" ValidationGroup="Login1" />
                                         </td>
                                         <td>
                                             <asp:Button ID="Cancelbtn" runat="server" Text="Cancel" />
                                         </td>
                                     </tr>
                                 </table>
                             </td>
                         </tr>
                     </table>
                 </asp:Panel>

             </td>
         </tr>