Steps guidance kindly provided by Frank Wall on Twitter
Tested on OPNsense 20.7.8_4 (may work on newer versions)
fetch https://pkg.opnsense.org/FreeBSD:12:amd64/21.7/latest/All/acme.sh-3.0.0.txz
fetch https://pkg.opnsense.org/FreeBSD:12:amd64/21.7/latest/All/os-acme-client-3.1.txz