Custom Frontmatter in Admin Plugin?

I created a custom Header in site.yaml called 'subtitle'.
Now I would like to show this in the Admin plugin under the 'site' settings where the title is.

How can I do this?

In order to add a new input field in the Admin Plugin user interface you'll need to define some extra theme blueprint.

Thanks, but I still don't get it all...
I checked the folder '\user\plugins\admin\blueprints\config' and see only a media.yaml file and if I understand correctly this is the place where the site.yaml file should be...?

Do I have to create one? If yes, with what content?

Hi, you need to extend the site blueprint