Thank you Robert, I had not found the first one. Unfortunately there seemed to be no sulution to the Problem...
What I now did as a workaround is to modify the print widget so I can manually input the custom text elements.
While this works in general it has the drawback that my co-workers can't use this new widget in the built-in WAB and that I have to host the whole application on our Webserver only because of this - so any ideas concerning the original problem would be appreciated.