MikroTik IPv6 DNS DHCPv6 Option Generator

In Winbox or WebFig, go to IPv6 > DHCP Server > Options and add a new option with code 23 and the following value:

0x2001148fffff000000000000000000012001148ffffe00000000000000000001

... or paste this piece of config to your router's terminal:

/ipv6 dhcp-server option 
add name=dns code=23 value=0x2001148fffff000000000000000000012001148ffffe00000000000000000001

After that, you just need to link the option with your DHCPv6 server entry and you're done!