You should be able to do it by linking the synergy.desktop file into your ~/.config/autostart/ folder by running the following command:
ln -s /usr/share/applications/synergy.desktop ~/.config/autostart/synergy.desktop
EDIT: I know this was written a while ago, but you can also just create a link to the file to the /etc/xdg/autostart so that it starts before logging in. Probably best to not have it in both home and the xdg autostart locations at the same time so it doesn't start more than once:
sudo ln -s /usr/share/applications/synergy.desktop /etc/xdg/autostart/synergy.desktop
Now when you reboot your host it should auto-start synergy before the login.
Hope this helps!