Tested using…

# uname -a
FreeBSD server 13.2-RELEASE-p2 FreeBSD 13.2-RELEASE-p2 GENERIC amd64

Create the jail for plex.

bsdinstall jail /usr/jail/plex

No services should need to be enabled. Create a root user with password root. When prompted create a plex user with password plex using defaults for all other settings.

Create an entry for the jail at /etc/jail.conf with the following contents. 

plex {
    host.hostname = "plex";
    path = "/usr/jail/plex";

    exec.clean;

    # Use whatever IP you prefer
    ip4.addr = 192.168.0.120;

    exec.prestart += "mount -t nullfs -o ro,nofail /mnt/my/mount /usr/jail/plex/multimedia";
    exec.poststop += "umount /usr/jail/plex/multimedia";

    exec.start += "/bin/sh /etc/rc";
    exec.stop = "/bin/sh /etc/rc.shutdown jail";
    exec.consolelog = "/var/log/jail_plex_console.log";
    mount.devfs;
    devfs_ruleset = "11";
}

Start the jail

service jail start plex

Verify the jail is running

# jls
   JID  IP Address      Hostname                      Path
     1  192.168.0.120    plex                          /usr/jail/plex

Start a shell in that jail using the JID returned by jls. 

jexec 1

Configure jails to run at boot time on the host if not already done by running the sysrc command like so. 

sysrc jail_enable=YES

Install Plex in the jail.

(inside the jail) $ pkg install plexmediaserver

Enable plex in the jail.

(inside the jail) $ sysrc plexmediaserver_enable=YES

Notes