To achieve your requirement you can create custom KeyboardCommandProvider and override the ProvideCommandsForKey method. Then you can double check if what is the current tab and add the BeginEdit command. I have prepared a sample project which demonstrates this behavior.
As a side note. In order to force the TextBox to accept Tab key to add an indent, the AcceptsTab property needs to be set to True. To do that you can create custom DataTemplate for the CellEditTemplate and place inside a TextBox. Check the attached project.
Regards,
Dinko
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