It's probably the URL in that account_id.json file, yes, assuming it looks like the URL in the documentation that starts with https://acme-v02.api.letsencrypt.org/acme/acct/ followed by a number.
From the announcement,
We'll be processing allow-list requests in batches, most of which will go out Thursday with our normal Production updates.
So you'll get an email once they update the list in their server configuration. Right now it seems to be a pretty manual process for the Let's Encrypt staff, and I'm guessing there's a lot of scrutiny around them changing their configuration files for the servers.