Drupal 8, create link to current page.

importForm.php

      $form['preview']['cancel'] = [
        '#type' => 'link',
        '#title' => $this->t('Cancel'),
        '#url' => \Drupal\Core\Url::fromRoute('<current>'),
        '#options' => [
          'attributes' => [
            'class' => [
              'btn',
              'btn-primary',
              'btn-one-row',
            ],
          ],
        ],
      ];