How to flatten TimeStampSpec Column

Hi,
I have a scenario where timestamp column is inside a json,like the below ones.

{

“version”:“1.1”,

“header”: {

“name”: “PlaceOrder”,

“schemaVersion”: “1.2.0”,

“appName”: “data-services-api”,

“appVersion”: “1.0.0”,

“experimentId”: “”,

“uuid”: “27811b74-2333-494d-9623-00d36cbefb9b”,

“eventId”: 35299,

“timestamp”: 1528351708570

}

}

Does anyone know how to specify this in the TimeStampSpec column of the config file?

“timestampSpec”: {
“column”: ???,
“format”: “millis”
}