I have to catch the event when I pressed on Shift Tab in TextBox to write some code. It is possible to do that? I tried with that test on KeyUp event : 
    private void txtJustifTampon_KeyUp(object sender, KeyEventArgs e)
    {
     if (e.KeyCode == Keys.Tab && Control.ModifierKeys == Keys.ShiftKey)
        {
            //do stuff
        }            
    }