AJAX dropbutton link
I assume these are links to Ajax enabled route yes ?? i hope so lol any way heres is how ... see this example from a list builder:

$parent['operations']['data']['#links']['clone'] = [
  'title' =&gt; $this-&gt;t('AJAX!!!'),
  'weight' =&gt; 2,
  'url' =&gt; Url::fromRoute('someroute_name', ['id' =&gt; $entity-&gt;id]),
  'attributes'=&gt; [
    'class' =&gt; 'use-ajax'
  ]
];


all you need to do is add 
'attributes'=> ['class' => 'use-ajax']
more info see my old comment ... https://api.drupal.org/comment/62725#comment-62725