Hello!
As mentioned in
https://forum.plantuml.net/11406/loadjson-function (in the PR
https://github.com/plantuml/plantuml/pull/755), the authentication to URL endpoints was not in the scope of the implementation for %loadJSON.
I started a discussion thread on GitHub (because the forum is currently very unstable):
https://github.com/plantuml/plantuml/discussions/783
Best regards, Aljoscha