* reference: Some clients and by far not all do understand nip-21 urls referencing nip-19 bech32 encoded public keys such as `nostr : npub...` (without the spaces).

* mention: All social clients understand `# [0]` as a mention of that pubkey in the first tag (index 0).

Currently, clients wildly differ in how they show mentions/references during editing. Most assist in finding your follows when you hit the @ key and start typing and show you some `@ npub...` but once you send, they replace it with some `# [index]` and add the actual pub key in the tags. Producing references is currently not well supported in any client I know of but you can always find the `npub` and prepend it with a `nostr:`