Retriving value from popup window

Hi..I am new to asp.net.PLz help me
I have a Textbox and a Hiddenfield, a linkbutton in my UserRegisteration.aspx page. 
when click on linkbutton a popup is opened with a dropdownlist .
I want how to get the selcteditem's text in textbox   and selected item's value in HiddenField after choosing an item from dropdownlist and closing the popup window.
 
Thanks a lot in advance.
Akhil Gupta
 
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