ssh ubnt@<your-ip>
configure
set service dns dynamic interface eth0 web https://ipinfo.io/json
set service dns dynamic interface eth0 web-skip "ip: "
First of all, create the subdomain in the Cloudflare zone.
set service dns dynamic interface eth0 service custom-cloudflare host-name <hostname.yourdomain.org>
set service dns dynamic interface eth0 service custom-cloudflare login <user@domain.com>
set service dns dynamic interface eth0 service custom-cloudflare password <CloudFlare API key>
set service dns dynamic interface eth0 service custom-cloudflare protocol cloudflare
set service dns dynamic interface eth0 service custom-cloudflare options zone=<yourdomain.com>
commit
save
https://help.ubnt.com/hc/en-us/articles/204976324-EdgeRouter-Custom-Dynamic-DNS
If you want to set multiple options you have to use double quotes. Example: