If your domain is hosted by Bind DNS server, you can add DKIM public key record like this:
Locate your domain's zone file and open it with your preferred editor.
then add the following content:
; DKIM policy record
_domainkey.yourdomain.com. IN TXT "o=~;"
; DKIM public key record
s1024._domainkey.yourdomain.com. IN TXT "k=rsa;p=MIGfMA0GCS....ofIz0IW7QwIDAQAB"
See Also:
Set DKIM record in Windows DNS server
