How do I force the dropdown list for a combobox to display?

I have a case where when the user selects a given checkbox, focus will move to a combobox from
which they are to select a value. We want that combobox to drop down automatically.

The only thing I have ever heard of is to use Keyboard F4 -- but I can't use that because F4 is already
assigned to as part of the system menu, to a feature that is used by everybody all the time.

Any other ideas?