Comment 3 for bug 1756630

This is not a fix.

But you can manually add a startup entry by hand from an lxterminal window. I do this as part of my install script. It works for 16.04 and 18.04 and might help someone in the mean time until this gets properly fixed:

manually create the necessary directories:
USERNAME=whateveryourusernameis
LXDE_AUTOSTART_FILE="/home/$USERNAME/.config/lxsession/Lubuntu/autostart"
mkdir -p "$(dirname "$LXDE_AUTOSTART_FILE")"
chown -R "$USERNAME:$USERNAME" "/home/$USERNAME/.config/"

appending your command to the autostart file:
CMD_TO_RUN=/path/to/whateverprogram
echo "$CMD_TO_RUN" >> "$LXDE_AUTOSTART_FILE"