You can add it manually to the webassist code as long as you know the values to add.
The syntax is:
$WA_Emal->Mailer->DKIM_domain = ‘mywebsite.com’;
$WA_Emal->Mailer->DKIM_private = dirname(__FILE__).’/key.private’; // Make sure to protect the key from being publicly accessible!
$WA_Emal->Mailer->DKIM_selector = ‘selector1-mywebsite-com._domainkey.mywebsite.onmicrosoft.com’;
$WA_Emal->Mailer->DKIM_passphrase = "";
$WA_Emal->Mailer->DKIM_identity = 'noreply@mywebsite.com';