The plugin is old, and the No-IP part was no doubt written long before it provided IPv6 support. Only a few providers are given IPv6 support in the plugin (https://github.com/opnsense/plugins/blob/master/dns/dyndns/src/etc/inc/plugins.inc.d/dyndns/phpDynDNS.inc).Judging from this thread (https://github.com/opnsense/plugins/issues/1611#issuecomment-706129596) it seems unlikely that it will be updated any time soon.
* No-IP - Last Tested: 20 July 2008
curl "https://<username>:<password>@dynupdate.no-ip.com/nic/update?hostname=<hostname>&myipv6=<ipv6address>&myip=<ipv4address>"
curl: (3) URL using bad/illegal format or missing URL
#!/usr/local/sbin/opnsense-shellIPV4=`curl https://ipv4.icanhazip.com/`IPV6=`curl https://ipv6.icanhazip.com/`curl \"https://<username>:<password>@dynupdate.no-ip.com/nic/update?hostname=<hostname>&myipv6=${IPV6}&myip=${IPV4}\"
#!/bin/sh# just in case the other shell doesn't work: /usr/local/sbin/opnsense-shellipv6=`curl https://ipv6.icanhazip.com`curl -X POST -u "<username>:<password>" https://dynupdate.no-ip.com/nic/update?hostname=<hostname> -d myipv6=${ipv6}