I have puzzled over this in the past - but gave up. The problem is that 'auto-tabbing' is a bit unconventional in Windows.
As far as I got (before I gave up) was to find on MSDN that there was a SetFocus function so that you could set the focus to any control for which you had the handle (MSDN talks about windows, but a control is also a window in MSDN language!). You could call this in the callback after you checked that the 2 characters were admissable, and get the [color=red:42a0ce1288]handle[/color:42a0ce1288] for every control with %lc.
However (and this is what really stopped me in my tracks) was the note that said not to use the function, but instead, CALL SendMessage so that internal bookkeeping was updated.
Perhaps you will get further than I did by Googling 'SetFocus' and 'MSDN' together!
Eddie
EDIT: When I said 'get the focus', I meant 'get the handle' - sorry. Corrected above in red.