Want to add some text in Header

I want to add some Text at this red case.
The text should be as normal login user and as guest user with a link.

Do you know what i mean?

Thank you for your help.

A proper solution would be to build an app that can do this... in this case we should move to the development category. Perhaps this can serve as inspiration: https://apps.owncloud.com/content/show.php/AgreeDisclaimer?content=170844

Or just some dirty css tricks directly in the code... or into a template?

An alternative could be to use a theme https://doc.owncloud.org/server/9.1/developer_manual/core/theming.html?highlight=theme

I did not find the .php file what i need for this. Can you help me?

I want to change it in the code but i did not find the right code snippet.

Here it is but i can not change it for all sites: