Code snippet that can be used to add link field to form in drupal 8.
$form['actions']['preview'] = [
'#type' => 'link',
'#title' => $this->t('Preview'),
"#weight" => 1,
'#url' => Url::fromRoute('content.newsletter.preview'),
'#attributes' => ['target' => '_blank', 'class'=> 'button button--primary js-form-submit form-submit'],
];
