I have been trying to figure out how to send a DM using #ActivityPub but I have still not found any clear instructions on what the message should contain for it to be a proper DM.
I believe it's a list of actors in the to field and these actors being included as a mention in tags.
But not 100% certain. It's all very awkward and very specific to how one writes one's front end code. I would expect that if someone were to write a messenger app for the Fediverse, e.g. something to talk with other people using DMs, its behavior would be different from the current implementations.
DMs are AFAIK a Mastodon feature, so I'm of course talking about Mastodon.
The Mastodon docs about how it interacts with ActivityPub can often be a good start for referencing AP in general, and this is one of those cases.
Here's how Mastodon handles DMs:
> direct: Mentions-only statuses have actors in to or cc, all of which are Mentioned in tag
