ContactFormEditForm::form

public ContactFormEditForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $form = parent::form($form, $form_state);

  $contact_form = $this->entity;
  $default_form = $this->config('contact.settings')->get('default_form');

  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Label'),
    '#maxlength' => 255,
    '#default_value' => $contact_form->label(),
    '#description' => $this->t("Example: 'website feedback' or 'product information'."),
    '#required' => TRUE,
  );
  $form['id'] = array(
    '#type' => 'machine_name',
    '#default_value' => $contact_form->id(),
    '#maxlength' => EntityTypeInterface::BUNDLE_MAX_LENGTH,
    '#machine_name' => array(
      'exists' => '\Drupal\contact\Entity\ContactForm::load',
    ),
    '#disabled' => !$contact_form->isNew(),
  );
  $form['recipients'] = array(
    '#type' => 'textarea',
    '#title' => $this->t('Recipients'),
    '#default_value' => implode(', ', $contact_form->getRecipients()),
    '#description' => $this->t("Example: 'webmaster@example.com' or 'sales@example.com,support@example.com' . To specify multiple recipients, separate each email address with a comma."),
    '#required' => TRUE,
  );
  $form['message'] = array(
    '#type' => 'textarea',
    '#title' => $this->t('Message'),
    '#default_value' => $contact_form->getMessage(),
    '#description' => $this->t('The message to display to the user after submission of this form. Leave blank for no message.'),
  );
  $form['redirect'] = array(
    '#type' => 'path',
    '#title' => $this->t('Redirect path'),
    '#convert_path' => PathElement::CONVERT_NONE,
    '#default_value' => $contact_form->getRedirectPath(),
    '#description' => $this->t('Path to redirect the user to after submission of this form. For example, type "/about" to redirect to that page. Use a relative path with a slash in front.'),
  );
  $form['reply'] = array(
    '#type' => 'textarea',
    '#title' => $this->t('Auto-reply'),
    '#default_value' => $contact_form->getReply(),
    '#description' => $this->t('Optional auto-reply. Leave empty if you do not want to send the user an auto-reply message.'),
  );
  $form['weight'] = array(
    '#type' => 'weight',
    '#title' => $this->t('Weight'),
    '#default_value' => $contact_form->getWeight(),
    '#description' => $this->t('When listing forms, those with lighter (smaller) weights get listed before forms with heavier (larger) weights. Forms with equal weights are sorted alphabetically.'),
  );
  $form['selected'] = array(
    '#type' => 'checkbox',
    '#title' => $this->t('Make this the default form'),
    '#default_value' => $default_form === $contact_form->id(),
  );

  return $form;
}