I couldn't figure it out either, but you could put another textbox under the one you are working with(by under I mean same top,left,height and width)and set it active and inactive with the one you are working with, then insert some code like this.
Private Sub txtHidden_GotFocus()
txtYouWantToIgnoreTabKey.SetFocus
End Sub
Not pretty, but it works.(I tried it)
Make sure hidden textbox is next in taborder.