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
pkg install acme.sh-3.0.0.txz
pkg install os-acme-client-3.1.txz
opnsense-patch -c plugins ae69739 9220a41 247408e 774374a4
service configd restart
On WebUI, go to Services: ACME Client: Certificates and click on the '*' (Re)Import button (4th from left to right); Restart the services that are using the certificates (ex. Nginx / HAProxy)