Ah, great! Did not need to add it into package.json for back4app. Sadly, this moved me to other error with the fetch:
error fetching session TypeError: Only absolute URLs are supported
I see that in the original post on stackOverflow were non-absolute URLs used, but above with the axios example it seems so.
Does this mean I should hardcode the server URL in the cloud code? Or is there any other “convert to string” trick?