Flex messages (also known as card-type messages) are a message format that can be delivered in any layout. Please refer to the LINE Developers documentation for more details on the specifications.
In order to deliver Flex messages, you will need to create JSON data, which can be created using the "LINE Bot Designer" provided by LINE.
However, in the case of Flex messages, copying the generated JSON code as is will not work. Some information is missing, and must be copied and used in the "contents" key of the JSON code below.
"altText": "This is a Flex Message",