DKIM and subdomains

Just add another DKIM TXT record for your subdomain and add the DKIM signature to it like this:

TX      abc._domainkey         v=DKIM1; lorem ipsum;
TX      abc._domainkey.mail    v=DKIM1; lorem ipsum;

I would recommend you add separate SPF and DMARC records for your subdomain as well. They should look something like this:

DMARC:

TX    _dmarc         v=DMARC1; lorem ipsum;
TX    _dmarc.mail    v=DMARC1; lorem ipsum;

SPF:

TX    @       v=spf1 ip4:12.34.56.789 -all;
TX    mail    v=spf1 ip4:12.34.56.789 -all;