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:[email protected]/nic/update?hostname=mydoma.in&myip=<ip6addr>
But I can't figure out the naming rule for IPv6 in the DSM DDNS settings.