What is the syntax for experimenter extension match-field and action in JSON file



I am trying to add some experimenter extension match-fields and actions for openflow protocol. The test method I use is to write JSON file for table addition with curl command for ODL. But I am not sure the syntax for experimenter extension in JSON file. 

For example, I have the following standard match fields. And I want to an experimenter match-field named  traffic-class. How should I write it in this JSON file? 
"match": {
      "ethernet-match": {
        "ethernet-type": { "type": "2048" },
        "ethernet-destination": { "address": "00:01:02:03:04:05" },
        "ethernet-source": { "address": "00:00:00:00:00:00" }
      "vlan-match": {
        "vlan-id": {
          "vlan-id": "10",
          "vlan-id-present": "true"



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