I have a downstream DHCPv6-PD client. RouterOS DHCPv6 relay has 0 documentation and does NOT work at all.That is an entirely different topic. For DHCPv6-PD from your provider you use DHCPv6 client, not relay.
Just configure it, specifying the interface name of the link to your provider (PPPoE interface if it is used)
and enter some arbitrary name for the pool. The client will request the prefix and put it in the pool.
Under IPv6 addresses assign the internal interfaces (LAN) an address from this same pool.