If you activate DKIM…
the selector is default by default.
You can check it by sending a mail to a different mail server (eg. Google Mail) and check the header.
Within the configuration dialog, you get a DNS record to paste into your DNS-Server like
default._domainkey IN TXT ( "v=DKIM1; k=rsa; " "p=abcdgefg" "hijklmn" ) ; ----- DKIM key default for yourdomain.de