What is the `Authorization` part of the http post request of Google's Firebase Downstream message?
<p>I want to try to send a message using google's <code>FCM</code> messaging service and as the document says, the http request should be something like this: </p>

<pre><code>https://fcm.googleapis.com/fcm/send
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA

{ "data": {
    "score": "5x1",
    "time": "15:10"
  },
  "to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1..."
}
</code></pre>

<p>My problem is that I have not idea what <code>Authorization</code>'s value should be and when I delete this header and make the request, I get error <code>401:Unauthorized</code>.I think this must be some kind of api key or something but I cannot find it in my project. Can anyone help me?</p>

<p>PS: for testing purposes I currently am using <a href="http://requestmaker.com/">this site</a> to send a messsage to my device</p>