There is no way for anonymous users of Keybase to send (encrypted) chat or emails to registered users of Keybase

There's already an excellent UI for taking some arbitrary plaintext and encrypting it for the owner of a given public key via the web, but there's no way to send that as a message to an existing Keybase user unless a) you have some external email service and you're willing to do the copy+paste+email dance or b) you have a Keybase account and you're using Chat.

Why not add a Web UI for sending a chat message without requiring login?

FYI, I perused the API docs to see if I could accomplish this myself with the provided APIs, but I didn't see this anywhere. I'd be happy with an API that does this even without a web UI.

Start by reviewing the existing web UI that encrypts plaintext for a public key and the API documentation referenced in the issue. Determine whether an unauthenticated API or web flow is the intended direction, including how a registered recipient is identified. Done means anonymous users can send encrypted chat or email to a Keybase user without external copy-and-paste steps.

Written by the indexing model from the issue text.