I followed the instructions and successfully generated a token using my login, but every call to the enrichment service led to a 498 invalid token error. I see I'm not the only one with this problem, but nobody bothered posting a solution.
The first problem is that there are two types of accounts, and I signed up for the ArcGIS online account, which the website guided me to. However, there is also a developer account (or capability) I needed to add. The hint for this was a link in the docs to a developer plan - but the link is broken. But it also said an organization plan (which I believe I signed up for) would work.
After that, I needed to create an application from within the developer interface.
Finally, I had to generate a token using Oauth 2.0. This is a completely different method of authorization from what the REST API docs state. I had to find this documentation here: