But when I try it via a HTML-widget the sidepanel does not appears like in the demo on the side and is also not fixed. Does anybody have an idea for me how to integrate it correctly?
You may also be interested in services such as AddThis , ShareThis , AddToAny , etc., most of which have simple setups, documented instructions, offer various customizations and advanced features. I believe all are free or offer a free version. You would implement those via the HTML Widget Bric and Page Settings also.