Although FreeNAS (TrueNAS) does support Dropbox backup, it is not the official Dropbox client that offers native and almost-real-time syncing.
Their new project, TrueNAS SCALE, is the Linux version of FreeNAS. Since it is Linux Debian OS rather than FreeBSD, you now can run any Linux programs like the official Dropbox client.
This guide is intended to learn how to run the official headless Dropbox daemon on bootup.