Your token, which will be a json document, will include the additional claims. E.g.:
"https://leanangle.io/claims/uuid": "abc123"
In the same way it includes standard claims:
"given_name": "Romain",
The string “https://leanangle.io/claims/uuid” is just the key in the key-value pair. Custom claims should always use a custom namespace to avoid conflicting with standard claims. By convention we use a URL-formatted string for the namespace, though it does not need to be a resolvable URL.
I would also suggest checking out these vue.js related blog posts. The Auth0 content engineers do an excellent job of providing very detailed walk-throughs.