What in your mind is the difference between a "control" and a "field"? I explained how focus works. And you had two suggestions of how to do what you asked but now you're saying that isn't what you want because you think "control" and "field" are different in this context.
If your tab order is correct, the user can tab to the first data entry field although that makes sense only if somehow the first record displayed is always the one the user wants to edit.
If the combos are unbound and used for searching, seems to me that it makes far more sense for focus to be in the combo when the form opens.