User-1334136856 posted
I found the following article on codeproject
http://www.codeproject.com/Articles/42456/ModalPopupExtender-to-Show-a-MessageBox?msg=4278164#xx4278164xx
which answered my questions.
What I needed, is the technique of adding events, so that I am able to continue in my page that calls the modal popup after pressing YES or NO.
The project example there continues in default.aspx
void omb_OkButtonPressed(object sender, EventArgs args)
{
lblStatus.Text = "Ok button pressed at: " + DateTime.Now.ToLongTimeString();
}
and already a button OK1 is prepared in the project, which can be changed quite easily into a NO or CANCEL button.