How do I set/get third party settings on a content block? I have a content block type, and I'd like to add a couple of simple configuration options to it without setting up "real" fields. I primarily used this for instructions:

https://www.webomelette.com/drupal-8-custom-data-configuration-entities-using-thirdpartysettingsinterface

My module code:

use Drupal\block_content\Entity\BlockContent;
use Drupal\Core\Form\FormStateInterface;

function oa_blocks_form_block_content_slider_images_edit_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  $block = $form_state-&gt;getFormObject()-&gt;getEntity();

  $form['autostart'] = [
    '#type' =&gt; 'checkbox',
    '#title' =&gt; t('Autostart'),
    '#default' =&gt; $block-&gt;getThirdPartySetting('oa_blocks', 'autostart'),
  ];
  $form['#entity_builders'][] = 'oa_blocks_block_content_slider_images_form_add_form_builder';
}

function oa_blocks_block_content_slider_images_form_add_form_builder($entity_type, BlockContent $block, &amp;$form, FormStateInterface $form_state) {
  if ($form_state-&gt;getValue('autostart')) {
    $block-&gt;setThirdPartySetting('oa_blocks', 'autostart', $form_state-&gt;getValue('autostart'));
    return;
  }
  $block-&gt;unsetThirdPartySetting('oa_blocks', 'autostart');
}


I have the following in config/schema/oa_blocks.schema.yml:

block_content.type.slider_block.third_party.oa_blocks:
  type: mapping
  label: 'Slider Block'
  mapping:
    autostart:
      type: boolean
      label: 'Autostart'


I get the following error when I try to edit the block:

Fatal error: Call to undefined method Drupal\block_content\Entity\BlockContent::getThirdPartySetting() in /Users/dicksb/Sites/devdesktop/drupal8/docroot/profiles/mogov/modules/mogov/mogov_blocks/mogov_blocks.module on line 17


What am I doing wrong?
ThirdPartySettingsInterface is used by config entitiy types:

Interface for configuration entities to store third party information.
A third party is a module that needs to store tightly coupled information to the configuration entity. For example, a module alters the node type form can use this to store its configuration so that it will be deployed with the node type.

BlockContent is a content entity type, and doesn't implement the interface, hence the error.