Added "dynamic" page title right to '|'

Grav seems to concatenate title of pages with the main site title of the page.
For one particular page, I want to add some text to the right side e.g.:
How can I do this via twig?

Grav doesn’t actually impose anything on the site title. This is managed by the theme. For example in Antimatter theme this is accomplished in partials/base.html.twig with:

<title>{% if header.title %},{{ header.title|e('html') }} | {% endif %},{{ site.title|e('html') }}</title>

You can change this to display whatever you want