"buttons":[ { "type":"postback", "title":"My Story", "payload":"STORY_ID-53856", }, ]Similarly, you can start the media block too with external API, you can find the payload of the Media Block from Media Block Options on the left sidebar as the following image and paste the copied payload as the value of payload in the button object in your JSON like the following
"buttons":[ { "type":"postback", "title":"Media Block Title", "payload":"TEMPLATE_ID-19534", }, ]