What is the IPv6 address syntax?

Prompted by my thinking in #154

unless the main resolver is IPv4 and secondary IPv6 or vice versa.

I am trying to set my main DNS server as https://149.112.112.112/dns-query and secondary as https://[2620:fe::fe]/dns-query (yes, I am one of those who put them the other direction). The usual syntax is to put the IPv6 address in brackets to avoid confusing port number as part of the address, but Nebulo doesn't accept it or without [] calling it as invalid address.

I have DualStack Lite at home and on mobile, two of the three biggest (W)ISPS in Finland have IPv6 in some form and mobile is where IPv6 is the most adopted in the world from what I have understood. There are also some events with IPv6-only WLANs for testing how well software behaves with it, so I would like to have both protocols available for DNS servers (preferring IPv6, but as most of random public WLANs are IPv4-only I guess it should be the main one for now).