User defined message search in the JSON format

Dear Experts,

how to give the xpath in the content modifier for implementing user defined message search to JSON format. i am sending data from SAP in the form of JSON to 3party system through CPI. i want to filter the messages based on this field ( document number ) and i am not using any message mapping in the IFLOW. i don't want to convert into XML format. is it possible to implement user defined message search only in JSON format?

Thanks in advance.

Best regards,

Nagaraju