Won’t accept API keys

Never mind. I finally got it working. The callback address had me tripped up. My WordPress installation is in its own directory, so I had modified the callback address to
https://mywebsite.com/directory/wp-admin/admin.php

That didn’t work.
I finally did change it to https://mywebsite.com/wp-admin/admin.php as suggested in your guide (even though that address, on my website, gives a 404).

And it worked. I don’t really understand this, since there is no “wp-admin” at that address, but hey, it’s working so it’s all right I guess.

It might be helpful to add a note in the instructions that this URL must be exactly in the stated format — even if the site in question isn’t really located there.