Unlock package management

# sed -i .orig 's/enabled: yes/enabled: no/' /usr/local/etc/pkg/repos/local.conf
# sed -i .orig 's/enabled: no/enabled: yes/' /usr/local/etc/pkg/repos/FreeBSD.conf

Update & install the package

#pkg update
#pkg install zerotier

Add on /usr/local/sbin this script with correct permission (in my example zt.sh)

after the join on the last row put your network ID

zt.sh :

#!/bin/bash
ln -s /usr/local/var/db/zerotier-one /var/db/zerotier-one
/usr/local/sbin/zerotier-one -d /usr/local/var/db/zerotier-one
sleep 1
/usr/local/bin/zerotier-cli join 0123456789012345

Start it after the boot process

TASKS -> init/shutdown scripts

ADD script in the postinit