Customize error pages

Hi, I want to customize the error page, especially HTTP 403 for no permissions. The users should know where to reach out to in order to get access so I want to display contact information.
I’ve tried finding the corresponding html files in the shinyproxy.jar by unpacking it, but wasn’t successful. Any ideas how to display a custom HTML file in case of an error?

I’m running shinyproxy-2.3.0.jar on k8s.
Many thanks!