Flutter How to enable vibration on firebase push notification?

I am trying to enable vibration on firebase push notification. Currently, I am testing from postman and I am able to receive push notification. But vibrate is not working. Here is my payload.

{
    "to": "<device-id>",
    "priority": "high",
    "sound":"default",
    "notification": {
        "vibrate":"true",
        "body": "Body",
        "title": "Title"
    },
    "data": {
        "click_action": "FLUTTER_NOTIFICATION_CLICK"
    }
}

I also tried modifying "android":{"notification":...} but that didn't work as well.