ideally you should add the & char in the description of the verb that identifies the action you want to trigger. it is in the registry key associated to the application related to the file extension of the selected file (hum.. quite complicated)
the char following the & will become your hotkey as stated in the msdn doc here
(of course this link explains how to do it better than I can)
Verbs can also have a display string associated with them, which is displayed on the shortcut menu instead of the verb string itself. For example, the display string for openas is Open With. Like normal menu strings, including an ampersand (&) in the display string allows keyboard selection of the command.
but (at least on windows 10 where I've tried it) it doesn't work for me, or maybe i'm doing something wrong.
Unfortunately I don't have a windows 7 on hand so I cannot verify the behavior there