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Any_extractJSON
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Use extractJSON.xml to extract required data to variable.
Use extractJSONSetBody.xml set body with variables.
Use json_orders_tree.txt as input JSON from Postman send to API proxy.

Try without header content-type
Try with header content-type = application/json

Reference:
https://help.sap.com/docs/integration-suite/sap-integration-suite/extract-variables