Symfony Mailer and DKIM

I changed my Mailsystem to Symfony Mailer and want to implement DKIM signing now

$signer = new DkimSigner($key, $domain, $selector); 
$message = Yii::$app->mailer->compose();

How to bring things together? I could not find any Docs …

Thanks

Yii2 extension does not support that out of the box.
PR to the repo for support is much welcome