I wrote to Ben of Bubbles.town and he replied very kindly that until Lemmy sets up an Oauth flow, he can’t implement a login through Lemmy. Here is his response:

thanks for your kind mail and your question about Lemmy auth.

Lemmy login isn’t on the roadmap, and I don’t expect that to change. Bubbles signs you in with the OAuth flow that Mastodon introduced: your server shows you a consent page, and Bubbles only receives a token that confirms who you are. Mastodon, Pixelfed, GoToSocial and a few others offer that flow, Lemmy doesn’t. A Lemmy app logs in with your username and password, and I don’t want Bubbles to ever ask for or handle your password.

The workaround Grant mentioned (sending a code by direct message) runs into the problem he described. Most Lemmy servers can’t receive the kind of direct message that other Fediverse software sends, so the code would never reach you.

Hi @grant@toast.ooo and @threelonmusketeers and thanks for answering. I’m not familiar with the workings of authentication and how it relates to the different types of instances. I kinda thought that all fediverse instances had somehow settled/agreed on a compatible standard but it seems my understanding is too shallow 😑.

I just saw from their FAQ that they only support a specific type of instance:

Which platforms are supported?

Anything with Mastodon-compatible OAuth: Mastodon, Pixelfed, GoToSocial, Pleroma, Akkoma, Misskey, Sharkey, Hometown.

Bluesky, Lemmy, and Threads are not supported.