Obtain promotion to board (3,2)
I believe the answer is:
get on
I believe this is a double definition.
'obtain promotion' is the first definition.
(I've seen this before)
'board' is the second definition.
(I know that board can be written as get on)
'to' acts as a link.
(Other definitions for get on that I've seen before include "Board - prosper" , "Board vehicle or make progress" , "Obtain promotion" , "Advance; prosper" , "Make progress, do well in life" .)