IPv6 and PPPoE

Been testing IPv6 for quite a while. Have a Mikrotik PPPoE server setup and handing out IPv6 space like so.

In all ppp profiles:
dhcpv6-pd-pool=pool1

for pool:
/ipv6 pool
add name=pool1 prefix=2xxx:xxxx:x::/48 prefix-length=64

The problem is on some PPPoE users I want to statically assign them a /64 instead of dynamic. How do I do that in ppp secrets on the Mikrotik.

No one knows?