DSM DDNS Custom Variables IPv6

I can't figure out the naming rule for IPv6 in the DSM DDNS settings. I want to set up a DDNS with my Strato account. The problem is that the Strato preset only allows IPv4 forwarding: But I need **only** IPv6. So I figured I could simply create my own custom preset, but there seems to be no naming rule for IPv6. Only __MYIP__, which returns the IPv4. I've checked the help and did some Google work but with no success. On my router it works like this: https://mydoma.in:myplaintextpw@dyndns.strato.com/nic/update?hostname=mydoma.in&myip= But I can't figure out the naming rule for IPv6 in the DSM DDNS settings.