try this

 <?php
 
 namespace vendor\module\Block\Adminhtml\Importpincode\Edit\Tab;
 
 class Importpincode extends \Magento\Backend\Block\Widget\Form\Generic implements \Magento\Backend\Block\Widget\Tab\TabInterface
 {
 protected $_assetRepo;
 
 public function __construct(
 \Magento\Backend\Block\Template\Context $context,
 \Magento\Framework\Registry $registry,
 \Magento\Framework\Data\FormFactory $formFactory,
 \Magento\Framework\View\Asset\Repository $assetRepo,
 array $data = []
 ) {
 $this->_assetRepo = $assetRepo;
 parent::__construct($context, $registry, $formFactory, $data);
 }
 
 /**
 * Prepare form
 *
 * @return $this
 */
 protected function _prepareForm()
 
 {
 
 $path = $this->_assetRepo->getUrl("vendor_module::importsample/sample.csv");
 
 $form = $this->_formFactory->create();
 $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
 $storemanager = $objectManager->create('Magento\Store\Model\StoreManagerInterface');
 
 $fieldset = $form->addFieldset(
 'base_fieldset',
 [
 'legend' => __('Import Pincodes'),
 'class' => 'fieldset-wide'
 ]
 );
 
 $importdata_script = $fieldset->addField(
 'file',
 'file',
 [
 'name' => 'file',
 'label' => __('Upload File'),
 'title' => __('Upload File'),
 'required' => true,
 ]
 );
 
 $importdata_script->setAfterElementHtml("
 <span id='sample-file-span' ><a id='sample-file-link' href='".$path."' >Download Sample File</a></span>
 ");
 
 $this->setForm($form);
 return parent::_prepareForm();
 }
 
 /**
 * Prepare label for tab
 *
 * @return string
 */
 public function getTabLabel()
 {
 return __('Import Pincodes');
 }
 
 /**
 * Prepare title for tab
 *
 * @return string
 */
 public function getTabTitle()
 {
 return $this->getTabLabel();
 }
 
 /**
 * Can show tab in tabs
 *
 * @return boolean
 */
 public function canShowTab()
 {
 return true;
 }
 
 /**
 * Tab is hidden
 *
 * @return boolean
 */
 public function isHidden()
 {
 return false;
 }
 }

**above code running good**

[![enter image description here][1]][1]


 [1]: https://i.sstatic.net/AK0MH.jpg