Symphony Post a message to one existing stream.

Post a new message to the given stream. The stream can be a chatroom,,an IM or a multiparty IM. You may include an attachment on the message. The message can be provided as MessageMLV2 or PresentationML. Both formats support Freemarker templates. The optional parameter "data" can be used to provide a JSON payload containing entity data. If the message contains explicit references to entity data (in "data-entity-id" element attributes), this parameter is required. If the message is in Mess...