Question: I have a contact form on my Drupal 8 site and I would like to remove the preview button and customize the html for the submit button.

I've tried this in my theme:

```
function mytheme_form_contact_message_feedback_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  $form['submit']['#prefix'] = '<div class="contact-form-btn col-xs-12 col-md-10 col-md-offset-2 no-pad-left">';
  $form['submit']['#suffix'] = '</div>';
  $form['submit']['#value'] = 'Submit';
  $form['submit']['#title'] = 'Submit';

}

```

But that doesn't seem to change either the html wrapping it or the label on the button itself.

Also, if you have any advice on how to remove the preview button I'd appreciate it!

Answer: 
I'm not sure why it didn't work as I was doing it above - because that works for all the other fields on the form, but the solution posted [here](https://www.drupal.org/node/2689265#comment-10997513) works.
Just to spell out what worked for me which I got from the link in the answer above.

In my mytheme.theme file I put the following code and I could both remove the preview button and also change the submit button text.

```
function mytheme_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  if (in_array($form_id, ['contact_message_feedback_form', ])) {
    $key = ($form_id == 'contact_message_feedback_form') ? 'actions' : 'basic';
    $form[$key]['submit']['#value'] = 'My Submit Message';
    $form[$key]['preview']['#access'] = FALSE;
  }
}

```