Help with Merlin Firmware Router nextdns ssd command

You don't need to - the profile ID belongs to the nextDNS profile you want to use. It's 6 character in the url: https://my.nextdns.io/XXXXXX/setup, If you want all clients on the same profile, you just use that, if you want some on other profiles, you need a line for each client that says "profile MAC_ADDRESS=PROFILE. Here's my working nextdns.conf on Merlin:

profile MAC_ADDRESS1=PROFILE1
profile MAC_ADDRESS2=PROFILE2
profile MAC_ADDRESS3=PROFILE2
profile MAC_ADDRESS4=PROFILE2
profile MAC_ADDRESS5=PROFILE3
profile DEFAULT_PROFILE
log-queries false
report-client-info true
discovery-dns
hardened-privacy false
max-inflight-requests 256
auto-activate true
debug false
max-ttl 0s
mdns all
detect-captive-portals false
bogus-priv true
listen localhost:5555
control /var/run/nextdns.sock
setup-router true
cache-size 0
cache-max-age 0s
use-hosts true
timeout 5s

Multiple clients can be set to the same profile.  Any clients on the network that you don't list are sent to the DEFAULT_PROFILE.  The router will send whatever it uses for the client name.