activating a combobox

[Ron Noice]

I'm sure to win the "Twit of the Day" award for this, but cannot figure out how to automatically activate a combobox field (button, menu, whatever). I want to be able to select something from combo1, populate combo2 based on combo1 and then have combo2 automatically open without the user needing to click it. Everything is under control _except_ the automatic opening of combo2. How, please? Thanks.
.... Ron