if you are using a DE that follows the freedesktop standards you can put a .desktop file in ~/.config/autostart/. Eg make a file like:
Comment=Setup and launch Synaptics
and put it at /.config/autostart/synaptics.desktop
However, you can likely put your synaptics configuration in /usr/share/X11/xorg.conf.d/50-synaptics.conf or similar and your DE may have a built in way of launching syndaemon. What distro and desktop (or window manager) are you using?