The page provides examples of HTTP Sink templating.
In this case the converters are irrelevant as we are not using the message content to populate our message template.
The HTTP request body contains the value of the message, which is retained as a string value via the StringConverter.
Specific fields from the JSON message are substituted into the HTTP request body alongside some static content.
The entirety of the message value is substituted into a placeholder in the message body. The message is treated as a string via the StringConverter.
Fields from the AVRO message are substituted into the message body in the following example: