A very easy way to accomplish this is to install games/minecraft-server, but be sure to configure the port itself and set it up so that it'll run as a service. That will also install /usr/local/etc/rc.d/minecraft. Then copy the file to spigot, edit it accordingly and you should be all set.
Make sure you got the ports collection installed (/usr/ports). If not use: # portsnap fetch extract.
Configure it: # make config and change it from stand alone to service.
# make install clean
Then simply perform the steps I mentioned earlier.
You'd copy /usr/local/etc/rc.d/minecraft, just as I mentioned earlier. You copy it to the same directory yet with the new name spigot. Then edit it accordingly so that it doesn't use the same rc.conf variables anymore and also point it to your spigot jarfile.