flex.json

{
      "type": "flex",
      "altText": "This is a Flex Message",
      "contents": {
        "type": "bubble",
        "body": {
          "type": "box",
          "layout": "horizontal",
          "contents": [
            {
              "type": "text",
              "text": "Hello,"
            },
            {
              "type": "text",
              "text": "World!"
            }
          ]
        }
      }
    }