BlockContentTypeForm::form

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

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

  /* @var \Drupal\block_content\BlockContentTypeInterface $block_type */
  $block_type = $this->entity;

  if ($this->operation == 'add') {
    $form['#title'] = $this->t('Add custom block type');
  }
  else {
    $form['#title'] = $this->t('Edit %label custom block type', array('%label' => $block_type->label()));
  }

  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => t('Label'),
    '#maxlength' => 255,
    '#default_value' => $block_type->label(),
    '#description' => t("Provide a label for this block type to help identify it in the administration pages."),
    '#required' => TRUE,
  );
  $form['id'] = array(
    '#type' => 'machine_name',
    '#default_value' => $block_type->id(),
    '#machine_name' => array(
      'exists' => '\Drupal\block_content\Entity\BlockContentType::load',
    ),
    '#maxlength' => EntityTypeInterface::BUNDLE_MAX_LENGTH,
  );

  $form['description'] = array(
    '#type' => 'textarea',
    '#default_value' => $block_type->getDescription(),
    '#description' => t('Enter a description for this block type.'),
    '#title' => t('Description'),
  );

  $form['revision'] = array(
    '#type' => 'checkbox',
    '#title' => t('Create new revision'),
    '#default_value' => $block_type->shouldCreateNewRevision(),
    '#description' => t('Create a new revision by default for this block type.'),
  );

  if ($this->moduleHandler->moduleExists('language')) {
    $form['language'] = array(
      '#type' => 'details',
      '#title' => t('Language settings'),
      '#group' => 'additional_settings',
    );

    $language_configuration = ContentLanguageSettings::loadByEntityTypeBundle('block_content', $block_type->id());
    $form['language']['language_configuration'] = array(
      '#type' => 'language_configuration',
      '#entity_information' => array(
        'entity_type' => 'block_content',
        'bundle' => $block_type->id(),
      ),
      '#default_value' => $language_configuration,
    );

    $form['#submit'][] = 'language_configuration_element_submit';
  }

  $form['actions'] = array('#type' => 'actions');
  $form['actions']['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
  );

  return $this->protectBundleIdElement($form);
}