Is there a way to execute a script that inserts route for delegated prefix?

[Erik Andersen]

We have a mikrotik router that proxies DHCPv6 PD requests to our ISC 
DHCP server.

The mikrotik router does not insert a route when it proxies the response 
back (https://forum.mikrotik.com/viewtopic.php?t=133572&p=656591).

If I had the prefix, the prefix length, relay link-local address, and 
the peer address, I could script inserting the route. (I would think 
this should be a process that forks, to avoid issues with the 
synchronous nature of executing stuff from ISC).

The log seems to show what I need:

Relay-forward message from 2607:f248:1::c:4297:71db port 547, link 
address 2607:f248:102:10::1, peer address fe80::3e90:66ff:fe28:2db2
Advertise NA: address 2607:f248:102:10::db86 to client with duid 
00:03:00:01:3c:90:66:28:2d:b2 iaid = 1713909170 valid for 604800 seconds
Advertise PD: address 2607:f248:3ff:f000::/56 to client with duid 
00:03:00:01:3c:90:66:28:2d:b2 iaid = 1713909170 valid for 604800 seconds
Sending Relay-reply to 2607:f248:1::c:4297:71db port 547

What expressions in the ISC DHCPv6 config file do I need to collect 
those values (so I can them hook them up to a script)?

Thanks,

Erik Andersen