Where to put #Ajax Callback in webform twig file
You can create a form alter function in your module file and then add the callback property to the respective field 

$form['actions']['submit']['#ajax'] = [
  'callback' =&gt; 'callback',
  'event' =&gt; 'click',
];


See also


Alter Ajax callback on Webforms
How do I add an AJAX action on a select element in a ContentEntityForm to change the value of another select element
Update webform field value using AJAX realtime