There is no info to find anywhere on the internet about this. So here we go…

configure
set service dhcpv6-server shared-network-name net_Main_LAN_IPV6 subnet 2001:470:7074:21::/64 static-mapping j-m910q ipv6-address 2001:470:7074:21:df::17
set service dhcpv6-server shared-network-name net_Main_LAN_IPV6 subnet 2001:470:7074:21::/64 static-mapping j-m910q identifier 00-01-00-01-27-B2-F9-5F-6C-4B-90-77-47-88

since the completion after subnet also shows prefix delegation, that could probably be programmed too.

jm-lemmi@USG-Wohnheim# set service dhcpv6-server shared-network-name net_Main_LAN_IPV6 subnet 2001:470:7074:21::/64
address-range       name-server         nisplus-domain      sip-server-address  static-mapping
domain-search       nis-domain          nisplus-server      sip-server-name
lease-time          nis-server          prefix-delegation   sntp-server