It's done with Javascript window.open command. Use your favorite search engine to find "asp.net popup" and you will find much help about this for different type of popups, some have created different confirmation popup controls for example that are more friendly to work with than you having to reinvent this particular wheel.
Asp.Net server code itself does not operate in client environment, it only pushes processed data to be viewed in response to client server requests, and can read data of a client modified page when request is made. In relation this, a popup is initiated client side from the static client side page and does not make a server roundtrip. It is not possible for asp engine to make this kind of thing happen.