The meat is this, using bsky-sdk + atrium-api from crates:

    let agent = BskyAgent::builder().build().await?;
    agent
        .login("username.bsky.social", "X")
        .await?;
    let rt = RichText::new_with_detect_facets(msg).await?;
    let record = RecordData {
        text: rt.text,
        created_at: atrium_api::types::string::Datetime::now(),
        facets: rt.facets,
    };
    agent.create_record(record).await?;

Works well enough. It's a bit on the lower end of the "abstraction ladder", there might be more user-friendly libraries for doing this even easier now.