How can I programatically add the default AJAX throbber to a form submit?
It should be easily done like this:

$form['submit']['#ajax'] = [
  'callback' =&gt; 'MYMODULE_submit_throbber_callback',
  'progress' =&gt; [
    'type' =&gt; 'throbber',
    'message' =&gt; NULL,
  ],
];


And then return $form in the callback;