1. Write the following ASP.NET code in the code behind file:

C# 8.0 and .NET Core 3.0 – Modern Cross-Platform Development: Build applications with C#, .NET Core, Entity Framework Core, ASP.NET Core, and ML.NET using Visual Studio Code, 4th Edition
ASP.NET Core 3 and Angular 9: Full stack web development with .NET Core 3.1 and Angular 9, 3rd Edition
C# in Depth, 4th Edition

void Page_load(object sender, EventArgs e)
Button1.Attributes.Add( “onclick”, “popWindow()” );

2. The .aspx page shoud contains popup JavaScript function:

function popWindow(){
window.open(‘https://www.yourdomain.com/yourpage.aspx’, ”, ”);”);
<form runat=”server”>
<asp:Button id=”Button1″ onclick=”Button1_Click” runat=”server” Text=”Run”></asp:Button>

When you click on the button the new popup Window will appear with the desired URL address.