code: public static Track registerTrackingWebhook(String carrier, String trackingNumber, String metadata, String apiKey)
  			throws AuthenticationException, InvalidRequestException, APIConnectionException, APIException {
  		Map<String, Object> params = new HashMap<String, Object>();
  		params.put("carrier", carrier);
  		params.put("tracking_number", trackingNumber);
  		params.put("metadata", metadata);
  		return request(RequestMethod.POST, classURLWithTrailingSlash(Track.class), params, Track.class, apiKey);
  	}
docs: Register webhook for tracking shipment of a package. This corresponds to
  API defined in https://goshippo.com/docs/reference#tracks-create. Please
  note that the webhook where information will be posted need to be already
  provided in https://app.goshippo.com
  
  @param carrier
  Name of the carrier (like "usps") tracking the packing
  @param trackingNumber
  Tracking number provided by the carrier for a package
  @param metadata
  Generic information related to this tracking
  @return Track object containing tracking info