How can I programatically add the default AJAX throbber to a form submit? I'm trying to add the AJAX throbber in core to a form submit button, specifically the Private Message module new message page.

Here is my code:

function MYMODULE_form_privatemsg_new_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['actions']['submit']['#ajax'] = array(
'callback' =&gt; 'MYMODULE_submit_throbber_callback',
  );
}

function MYMODULE_submit_throbber_callback($form, $form_state) {
  return;
}


Here's what I have done so far.

First, I looked at the Page Throbber module for ideas, but this does everything in hook_init(), and it doesn't use the throbber included in core.  If possible, I want to avoid replicating what is already in core for a single form.

Next, I looked in the Examples module, specifically the AJAX example that includes an AJAX submit example.  However, this is for adding a callback to do something else, but all I want to do is get the pager, which appears to happen automatically if you add any kind of callback.

So I added a dummy callback that just returns.  This gives me a throbber, but it breaks the submit functionality of the form.  How do I get the throbber without breaking the 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;