Minecraft wiki entry for FreeBSD_startup_script, associated text: Use adduser to create a new user named minecraft. When asked to select the new user's home directory, use /srv/minecraft.
Ensure that your /srv filesystem has enough free space (around ~100-200MB depending on your needs) - df -h will show how much space is available.
Install Java if necessary. The port is available at /usr/ports/java/openjdk8.
As root, run the following: