Select

$form['button_target'] = [
   'type'    => 'select',
   'title'   => t('Target'),
   'tab' => 'section_content',
   'style_type'      => 'attribute',
   'style_selector'  => '.btn',
   'attribute_type'  => 'target',
   'options' => [
      '' => 'Opens in the same frame as it was clicked',
      '_blank' => 'Opens in a new window or tab',
      '_parent' => 'Opens in the parent frame',
      '_top' => 'Opens in the full body of the window',
      'framename' => 'Opens in the named iframe',
   ]
];