javascript event in content pages

 my content page has textbox. and i am populating some values in a listbox dynamically.
everytime i load a listbox i laod the balues, so i cannot use postback event handler to get the selected item.
 
So i had to use javascirpt event which will allow to make the text of textbox = listbox selected item.text
i tried this
javascript:document.getElementById('<%=txtSecretQstn.ClientID%>').text =  document.getElementById('<%=lstbox1.ClientID%>').selectedindex
...