Add code into email header

How can I add code into an email header?

Ideally at email level, but at least at template level.

Just seen that if you create a template and use the old editor you can ad code in the header.

Hi,

You also can add this with the drag & drop editor. However, for both it's available in templates only.

Please note that we also have an add-on available for custom headers. These will be applied to all email you send regardless of the template you use. It's socumented here: https://docs.acymailing.com/addons/all-cms-add-ons/custom-headers