Drupal - Submit form and disable submission button

For those wanting to disable the form submit button on load, try this:

$form['actions']['submit']['#attributes']['disabled']  = 'disabled';

In your context, this refers to the submit input element which doesn't have the method submit(). You can use some jQuery to disabled the element on click then find the form element to submit.

'#attributes' => array("onclick" => "
      jQuery(this).attr('disabled', true);
      jQuery(this).parents('form').submit();
"