How to add a custom button in header ?

[technikte]

Hello,
is there any easy way to add an button in the header between contacts and settings or is there maybe a plugin to add buttons ?
My goal is to add a link to my mailcow ui where I can edit alias and temp spam-alias and so on. A hyperlink which open in a new tab or a iframe inside the roundcube ui would be very nice.

I edited already the style.css of the larry skin and this how I add the text in screenshot but from there I dont really know how add all missing things for it.