Stopping Movieclip when clicking on Combobox

Hi there
I need some help on an issue that has puzzled me for some time. I am usinga Flash CS4 file using Actionscript 2.0
I have a movieclip and a button and a combobox next to it.

When I click on the button **btn **I use sucessfully the code below to stop the movieclip
btn.onPress() = function {

The question is
How do I stop the movieclip when clicking on the combobox?

I tried on(dropdown) or on(open) but no help!!!