It’s not undocumented because it’s open source, it’s undocumented because no one (or not many) have asked.
Suggestions that you want to POST the encoded email in an email_encoded thingy. routes.rb shows that it’s a POST and the route, which is https://discourse.example.com/admin/email/handle_mail. I got it from the mail receiver sample template (and I’d recommend that you just use that, as if you had, you’d be finished already) but it’s also in discourse/routes.rb at main · discourse/discourse · GitHub (which isn’t immediately clear just what’s happening unless you understand rals).