I’ve used a workaround to be able to display non-public calendar in an iframe widget in this app.
For this I’ve misued the server url function to load first google website. You’ll get an error message that habpanel html was not found (or something similar), but after clicking on the google logo at the top I was able to use google sign-in. Once the login was completed I’ve changed back to the server url and voila - iframe now also displays the embedded non-public calendar.
It may be useful for some people who are dealing with similar issues.