In the case of Microsoft Online Selector1._domainkey.domainname.tld should be a CNAME pointing to onmicrosoft.com where the actual DKIM Selector TXT record is published.
If so, return Selector1=selector1-domainname-tld._domainkey.domainname.onmicrosoft.com
In the case of Microsoft Online Selector1._domainkey.domainname.tld should be a CNAME pointing to onmicrosoft.com where the actual DKIM Selector TXT record is published.
If so, return Selector1=selector1-domainname-tld._domainkey.domainname.onmicrosoft.com