This is missing and annoying to do with the tool or the transform palette flyout. Pretty sure you can't even set your own shortcut within the program. Please add this standard.
In the meantime if you have autohotkey I made this script to map the alt menu to do it.
I've also added better blend shortcuts and rounded corners.
^ means control
+ means shift
! means alt
; means comment
#IfWinActive ahk_class illustrator
;Roundcorners with preview already checked