I have a feature request for Kirbytag Mail and the related helper. Why not add useful fields as subject, content…
You can create a feature request here: https://github.com/getkirby/ideas
But you can also create your own custom kirbytag.
I found an easier solution by adding wanted data to email
function:
$emailAddress = "hello@john.smith";
$emailSubject = "Move it!";
$emailBody = "Move your body to the rhythm of the night";
$myMail = html::email($emailAddress."?subject=".$emailSubject.'&body='.$emailBody,
'Email me!');
OK, I thought you wanted to enter this information in a KirbyTag, not in a template.