If you're sending in the first message of a conversation, you should send in a null conversationId and a null parentMessageId. This first call will generate a new conversation, and you will receive an id for the conversation in the response body.
At least one of externalId or email must be provided.
Any key-value pair is allowed.
Some keys are recognized by Pal and displayed separately in the UI.
For now, the only recognized key is sourceUrl, which Pal expects to be a URL string.