Expected OAuth 2 access token while importing VAPID Push subscriptions into Firebase

Question

I am trying to import vapid push subscriptions into firebase. I found some solution in https://developers.google.com/instance-id/reference/server#import_push_subscriptions. It thew following error. Please help me regarding this.

POST : https://iid.googleapis.com/v1/web/iid

Headers :
Content-Type:application/json
Authorization:key=MY_SERVER_KEY

BODY:

{
        "endpoint" : "https://fcm.googleapis.com/fcm/send/my_sub_key",
        "keys" : {
            "p256dh" : "hidden_key",
            "auth" : "hidden_auth"
        }
    }
}

Response:

"error": { "code": 401, "message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.", "status": "UNAUTHENTICATED" }