The correct declarative design is to use a Data Mapper Transform after the HTTP Action. The HTTP Action performs the REST call and returns the response to the Integration Procedure. If that response must be converted from JSON into a required XML structure, the transformation layer should be a Data Mapper Transform, because transforms are built to map input paths to different output structures and formats without writing Apex. XMLStreamReader is for reading XML, not building the target XML payload from JSON. XMLStreamWriter could be used in custom Apex, but the OmniStudio-native answer avoids unnecessary code. A Data Mapper Extract reads Salesforce data, so it is not the right tool for restructuring an HTTP response.

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