Render a link from a FieldPluginBase
You need to convert array to string to show it as link. May this works for you.

class MyCustomClassViewsField extends FieldPluginBase {
...
    public function render(ResultRow $values) {
      $node = $values-&gt;_entity;
$title = 'Custom Link';
$url = \Drupal\Core\Url::fromUri('http://www.test.com');
$link = \Drupal\Core\Link::fromTextAndUrl($title, $url)-&gt;toString();

      return $link;
    }