According to the system requirements FreeBSD is supported for the Universal Forwarder, but there doesn't appear to be a v8.1 package for it. The latest FreeBSD Universal Forwarder package is currently 8.06, which can be found on the Older Splunk Universal Forwarder Releases page.
https://www.splunk.com/en_us/download/previous-releases/universalforwarder.html#tabs/freebsd