How to Create modal Pop-Up form in asp.net using vb.net :

The ModalPopup create a modal popup using client-side means it's control in the AJAX Control Toolkit offers triggered on the server-side. use this below code for create model pop-up : 

<asp:Button ID="ClientButton" runat="server" Text="Launch 
 Modal Popup (Client)" />


<ajaxToolkit:ModalPopupExtender ID="mpe" runat="server"

 TargetControlId="ClientButton" PopupControlID="ModalPanel" 

 OkControlID="OKButton"/>

<asp:ScriptManager ID="asm" runat="server" />

<asp:Button ID="ServerButton" runat="server" Text="Launch Modal Popup (Server)"  
 OnClick="ServerButton_Click" />

<script type="text/javascript"> 
 var launch = false;

 function launchModal() 

 {

 launch = true;

 }

example: 

<form id="form1" runat="server"> 
    <div>

        <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">

        </asp:ToolkitScriptManager>

        <asp:Button ID="Button1" runat="server" Text="fill form" />

        <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" CancelControlID="Button2" TargetControlID="Button1" PopupControlID="Panel1">

        </asp:ModalPopupExtender>

        <asp:Panel ID="Panel1" runat="server" CssClass="Popup" align="center" style = "display:none">

        <iframe style=" width: 350px; height: 300px;" id="irm1" src="test2.aspx" runat="server"></iframe>

        <br/>

        <asp:Button ID="Button2" runat="server" Text="Close" />

        </asp:Panel>

    </div>

    </form>