code: public function getFieldConfiguration()
      {
          $customSettings = unserialize($this->CustomSettings);
          $content        = (isset($customSettings['Content'])) ? $customSettings['Content'] : '';
          $textAreaField  = new TextareaField(
              $this->getSettingName('Content'), 'HTML', $content
          );
          $textAreaField->setRows(4);
          $textAreaField->setColumns(20);
  
          return [
              $textAreaField,
          ];
      }
docs: Get extra configuration fields.
  
  @return array