As far as I'm aware this works as is in 6.38.5.
Do the following on the DHCP Relay
/ip dhcp-relay add add-relay-info=yes dhcp-server=10.1.1.1 disabled=no interface=br1 local-address=192.168.1.1 name=relay1 relay-info-remote-id=ether3
This tells the router to forward DHCP requests on the bridge interface br1 to the DHCP server at 10.1.1.1, reference the local address of the subnet, bridge, 192.168.1.1 and to include option 82 information of the client device MAC, the bridges MAC address and the relay information of ether3.
This can be viewed on the MikroTik DHCP server by
/ip dhcp-server lease print detail
Here is a dump of my lab in GNS3 for this
[admin@r1] > ip dhcp-server lease print detail
Flags: X - disabled, R - radius, D - dynamic, B - blocked
0 D address=192.168.1.199 mac-address=00:50:79:66:68:00 client-id="1:0:50:79:66:68:0" address-lists="" server=br1
dhcp-option="" status=bound expires-after=7m47s last-seen=2m13s active-address=192.168.1.199
active-mac-address=00:50:79:66:68:00 active-client-id="1:0:50:79:66:68:0" active-server=br1 host-name="PC11"
src-mac-address=00:0B:0B:CB:59:00 agent-circuit-id="11:11:11:22:22:22" agent-remote-id="ether3"
Now the problem here is that a MikroTik relay agent always sends the bridges MAC address and the same agent-remote-id value each time. So on the DHCP server side you can't do much.
To be more specific I imagine you are looking for the relay information to contain the individual interface within the bridge that it was relayed from like Cisco does with DHCP Snooping. Alternatively you may find sanction in RFC3993 or the subscriber ID option. I also know MikroTik likes having an RFC to implement from. This would allow you set a value that is carried along in the DHCP process at the relay on a per-interface basis and would be separate from trying parse the agent-circuit-id field in any way.
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