Typed too soon, I figured it out, but I’m guessing this is “unofficial”
curl --location 'https://MY_DISCOURSE.com/chat/4.json' \
--header 'Content-Type: application/json' \
--header 'Api-Key: THE_KEY' \
--header 'Api-Username: THE_ACTING_USER' \
--data '{ "message": "My great chat message!"}'