There appears to be a bug with trying to set external fetch URL permissions e.g.
The error is ‘There was an error invoking the function - URL not included in the external fetch backend permissions’
There is a small simple project to replicate the error at Bitbucket
I am running into the same issue and have posted my question here User consent related error with Forge app
This looks to be due to the way you are using
In the example provided you are supplying the URL and options as a single object where the method signature is
fetch(url[, options]) as per the reference.
Sorry our error messaging for this is a bit misleading, we’ll work on fixing that to help in the future.