clientId

The unique identifier of the MQTT client to send the message to.

Client IDs must not exceed 128 characters and can't start with a dollar sign ($). MQTT client IDs must be URL encoded (percent-encoded) when they contain characters that are not valid in HTTP requests, such as spaces, forward slashes (/), and UTF-8 characters. For more information, see Amazon Web Services IoT Core message broker and protocol limits and quotas.