create pop up window in ASP.NET

You may call the below function to create popup window at runtime

public void PopUp(string url){

string myPopup = "<script language='javascript'>" + "window.open('" + url + "', 'CustomPopUp', " + "'width=200, height=200, resizable=no')" + "</script>";

Page.RegisterStartupScript("Popup", myPopup);

}