The keypress
event is fired when a key is pressed down and that key normally produces a character value.
You can use the keydown
event instead:
ui-keydown="{ 'tab':'add_plu(order.orderwindow.add_field)'}"
Demo: http://plnkr.co/edit/XMRlVLnw2BEyoVgvQyW9?p=preview
Note that you need to click the button first to give it focus, which is another issue.