$ sudo apt-get update
$ sudo apt-get install openbox-xdgmenu 

openbox-xdgmenu is installed in your system.

Make ensure the openbox-xdgmenu package were installed using the commands given below,

$ sudo dpkg-query -l | grep openbox-xdgmenu *

You will get with openbox-xdgmenu package name, version, architecture and description in a table

openbox-xdgmenu package basic information: