wikipedia.org:
https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail

In DirectAdmin:

Configuration:
In /usr/local/directadmin/conf/directadmin.conf check or add: dkim=1
(you may remove the two domain key lines in the DNS inside DirectAdmin if already exist)

Retrieve code in DirectAdmin by subdomain (manually):
# cd /usr/local/directadmin/scripts
# ./dkim_create.sh example.com
or
# ./dkim_create.sh sub.example.com

Identical result for a DNS and for a DNS of a sub domain in DirectAdmin:
_domainkey TXT “o=~”
x._domainkey TXT “v=DKIM1; k=rsa; p=…”

DNS on the name server for a domain and for a sub domain:
(subdomain resell2 is an example):
_domainkey TXT o=~
_domainkey.resell2 TXT o=~
x._domainkey TXT v=DKIM1; k=rsa; p=…
x._domainkey.resell2 TXT v=DKIM1; k=rsa; p=…

Notes:
In order to avoid spaces, edit the key before pasting
Extra setup is in /etc/exim.dkim.conf