as per their docs you can go to postman click on import and then copy paste the following

curl --location 'https://graph.facebook.com/v18.0/<BUSINESS_PHONE_NUMBER_ID>/messages' \
--data '{
    "messaging_product": "whatsapp",
    "to": "<CUSTOMER_PHONE_NUMBER>",
    "recipient_type": "individual",
    "type": "interactive",
    "interactive": {
        "type": "flow",
        "header": {
            "type": "text",
            "text": "<HEADER_TEXT>"
        },
        "body": {
            "text": "<BODY_TEXT>"
        },
        "footer": {
            "text": "<FOOTER_TEXT>"
        },
        "action": {
            "name": "flow",
            "parameters": {
                "flow_message_version": "3",
                "flow_action": "navigate",
                "flow_token": "<FLOW_TOKEN>",
                "flow_id": "<FLOW_ID>",
                "flow_cta": "Open Flow!",
                "flow_action_payload": {
                    "screen": "<SCREEN_ID>",
                    "data": {
                        "<CUSTOM_KEY>": "<CUSTOM_VALUE>"
                    }
                }
            }
        }
    }
}'