This guide was written assuming you are configuring a remote machine, but can be used for a machine you have physical access to.
Change to root with
su -l and install
pkg_add -U syncthing.
syncthing to start automatically with
rcctl enable syncthing and (if you are working with a remote machine) temporarily override the default configuration with
rcctl set syncthing flags -no-browser -gui-address=0.0.0.0:8384 unless you'd prefer to use
ssh port forwarding.
rcctl start syncthing.
Go to the remote address, port 8384, and configure a username and password, as well as TLS. Otherwise, your
syncthing GUI will be accessible to anyone.
Then remove the extra flags from your
/etc/rc.conf.local file and restart
rcctl restart syncthing.