In case anyone else stumbles across this with similar trouble, it’s not at all obvious how this works.
This page will help greatly - there’s a video there that you can pause your way though, and there are paste-able examples from the video below it:
HOWEVER … none of that works (which is the topic of this original post) until, as the instructions say, “A Custom Page Template should be configured”.
However, how to do that is not obvious.
THERE IS NO GUI FOR IT. You have to the do it all with the Management API. This page (it’s really LOTS of pages) documents the Management API in general:
https://auth0.com/docs/api/management/v2/introduction
SPECIFIALLY, to satisfy the “A Custom Page Template should be configured” requirement to avoid the error in this post, look here:
https://auth0.com/docs/customize/login-pages/universal-login/customize-templates
…and scroll WAY down do " Page templates API".
It shows you 3 CURL commands for Setting, Retrieving, and Deleting the template.
Just run the “Set” one (you’ll need to paste your own API Management Token in there, which is shown in the video in the first link), and after that, your Management API calls will work to do things like add extra fields to the Sign On prompt:
https://auth0.com/docs/api/management/v2/prompts/put-partials
Hope all of this helps someone.