function sendFcmMessage(fcmMessage) {
  getAccessToken().then(function(accessToken) {
    const options = {
      hostname: HOST,
      path: PATH,
      method: 'POST',
      // [START use_access_token]
      headers: {
        'Authorization': 'Bearer ' + accessToken
      }
      // [END use_access_token]
    };

    const request = https.request(options, function(resp) {
      resp.setEncoding('utf8');
      resp.on('data', function(data) {
        console.log('Message sent to Firebase for delivery, response:');
        console.log(data);
      });
    });

    request.on('error', function(err) {
      console.log('Unable to send message to Firebase');
      console.log(err);
    });

    request.write(JSON.stringify(fcmMessage));
    request.end();
  });
}
Send HTTP request to FCM with given message.

@param {object} fcmMessage will make up the body of the request.