Good point. There was an example but it's gone now. I'll add it back. In short
Create a note with NostrEvent::getNote (see getEncryptedDm for example of params) and broadcast the string returned from that method
https://github.com/lnbits/arduino-nostr/blob/438574714a4b0bf41d3ce3e4b59cd06348c008e2/src/NostrEvent.cpp#L60