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;