/**
  * @param \Magento\Shipping\Block\Adminhtml\View $subject
  */
 public function beforeGetBackUrl(\Magento\Shipping\Block\Adminhtml\View $subject)
 {
     if ($subject->getRequest()->getFullActionName() == 'adminhtml_order_shipment_view' && $this->_shipmentHelper->isMercadoEnviosMethod($subject->getShipment()->getOrder()->getShippingMethod())) {
         $subject->addButton('custom_button', ['label' => 'Print shipping label', 'onclick' => 'window.open(\' ' . $this->_shipmentHelper->getTrackingPrintUrl($subject->getRequest()->getParam('shipment_id')) . '\')', 'class' => 'go']);
     }
 }