Write the following JavaScript function on your page like as follows:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>PopUp Window</title>
<script type="text/javascript">
function popup(url) {
var width = 300;
var height = 200;
var left = (screen.width - width) / 2;
var top = (screen.height - height) / 2;
var params = 'width=' + width + ', height=' + height;
params += ', top=' + top + ', left=' + left;
params += ', toolbar=no';
params += ', menubar=no';
params += ', resizable=yes';
params += ', directories=no';
params += ', scrollbars=no';
params += ', status=no';
params += ', location=no';
newwin = window.open(url, 'd', params);
if (window.focus)
{
newwin.focus()
}
return false;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
 <a href="javascript: void(0)" onclick="popup('http://www.c-sharpcorner.com')">Click Here</a> </div>
</form>
</body>
</html>

If you want to show some message or some content in popup window then you can give the xml file path like as follows:

<a href="javascript: void(0)" onclick="popup('XMLFile.xml')">Click Here</a>

Output:

popup.JPG
Figure: Popup window on Click 'Click Here' button.

Note: This will take some time because page will be load.