In the base system, load the daemon if you need it
/etc/rc.conf[.local]
ftpproxy_flags=""
/etc/pf.conf
anchor "ftp-proxy/*"
the redirection
pass in quick proto tcp to port ftp rdr-to 127.0.0.1 port 8021
pass out proto tcp from $proxy to port ftp
Note: from 5.0 onwards, ftp-proxy uses divert sockets, divert-to