CKEditor inline image files usage for custom forms.

Example with CKEditor WYSIWYG inline images field.

Also see : Create a Form with Drupal 8 using Form API

/**
* {@inheritdoc}
*/
public function buildForm(array $form, FormStateInterface $form_state)
{

   $form['text'] = [
       '#type' => 'text_format',
       '#format' => 'full_html',
       '#title' => $this->t('CKEditor text'),
   ];

   return parent::buildForm($form, $form_state);
}

/**
* {@inheritdoc}
*/
public function submitForm(array &$form, FormStateInterface $form_state)
{
   $text = $form_state->getValue('text')['value'];
   try {
     self::ckeditorSetFileUsage($text);
   }
   catch (EntityStorageException $e) {
   }
   
   // custom codes.
   
   parent::submitForm($form, $form_state);
}

/**
* Set Images / Filesas Permanent.
*
* @param string $text
*
* @throws \Drupal\Core\Entity\EntityStorageException
*/
 public static function ckeditorSetFileUsage(string $text, $module = 'ckeditor') 
{
   $uuids = _editor_parse_file_uuids($text);
   foreach ($uuids as $uuid) {
       if ($file = \Drupal::service('entity.repository')->loadEntityByUuid('file', $uuid)) {
           /** @var \Drupal\file\FileInterface $file */
           if ($file->isTemporary()) {
               $file->setPermanent();
               $file->save();
           }
           \Drupal::service('file.usage')->add($file, $module, 'file', $file->fid->value);
       }
   }
}