Unfortunately here is how the Tab key works. If there is only one control on a form with TabStop = True, the Tab key will be recognized and you can check for it, but if there is more than one you can't check for it. What you need to do is this:
1) In the form's Load event, store the TabStop values for all the controls that you are interested in
2) Then in the GotFocus even of each control set the TabStop property of all other controls to False (you could write a subroutine to do this)
3) And then in the LostFocus event set the TabStop values of all the controls back to their originally stored values.