How to add NDEF message to mqtt.publish_json

Hello
have “up and running” a PN532 tag reader:

pn532_i2c:
  id: pn532_board
  on_tag:
    then:
    - mqtt.publish_json:
        topic: pn532/tagJson
        payload: |-
          root["key"] = x;
          root["greeting"] = "PN532: Hello World";

how can the NDEF message be added to the MQTT publish_json command in addtional to the UID ?
Thanks
Norbert