PPPoE Sever DHCPv6 DNS Server

Hi,
I want to configure specific PPPoE clients with an IPv6 DNS server address which is different from the address under /ip dns.
For IPv4 this can be achieved easily by setting an alternative DNS Server in /ppp profile.

But how to configure the dynamic DHCPv6 server for the PPPoE sessions to send out an alternative IPv6 DNS server address?