add_subscribe_header.json

[
  {
    "id": 1,
    "priority": 1,
    "action": {
      "type": "modifyHeaders",
      "requestHeaders": [
        {
          "header": "Subscribe",
          "operation": "set",
          "value": "true"
        }
      ]
    },
    "condition": {"urlFilter": "*", "resourceTypes": ["main_frame"]}
  }
]