We already have a setup for Asistant plan with Messages: sample below:

"structuredDataPlan": {
            "schema": {
                "type": "object",
                "properties": {
                    "status": {
                        "type": "string",
                        "enum": [
                            "Yes",
                            "No"
                        ]
                    }
                },
                "required": [
                    "status"
                ]
            },
            "messages": [
                {
                    "content": "{\n  \"structuredDataPrompt\": \" You are an information extraction assistant, You will be given the transcript of a call between ...
                    "role": "system"
                }
            ],
            "timeoutSeconds": 10
        },