I'm using icewm specifically and I have downloaded directly from itch website, not the app so my method will not apply for you directly. However, your distribution will probably have some method of launching any program at login (~/.profile, gnome-session-properties gui settings menu, etc).
Download the program, make sure it is executable (run sudo chmod u+x on the deskspace.sh file) and follow whatever method you choose. Just be sure you provide a path to the file as it will not be in your $PATH and you can't just run it by typing "deskspace"