It's for an Auto Reply with no real acct named autoreply@. So DKIM authentication is working on emails sent directly from email. I did set it up on DNS records some time ago. However, I saw online an example somebody gave that was added to the PHPMailer code that looked like this. I was thinking it's probably something you've used in the past.
$mail->DKIM_domain = ‘mywebsite.com’;
$mail->DKIM_private = dirname(__FILE__).’/key.private’; // Make sure to protect the key from being publicly accessible!
$mail->DKIM_selector = ‘selector1-mywebsite-com._domainkey.mywebsite.onmicrosoft.com’;
$mail->DKIM_passphrase = "";
$mail->DKIM_identity = 'email@example.com';