How to show window using ajax to take input ?

ajax modalpopup extender control is there once you drag and drop that control into your page and sett the setting what ever you want for that you must add the ajax tool kit dll

the code for this is

     CancelControlID="lnkClose" BackgroundCssClass="modalBackground">
   

here
target control id is which control is trigger the showing  of the popup window
popupcontrolID : which is the control to show when the action is happen generally for this we take panel control in this panel we design as you like
canCelControlID : this property is used to set the control to  closed the open popupwindow 

backgroundCss: the css which is apply the main window when the popup is shown