# Create directory /etc/X11/sessions/ and install this there
[Desktop Entry]
Name=Custom X11 session
Comment=Configured in ~/.xsession or update-alternatives
# Exec=custom is special-cased by the Xsession script to run ~/.xsession
# or an x-session-manager or x-window-manager. It can be replaced by any
# command, for example Exec=openbox
Exec=custom
Icon=
Type=Application