Missing field 'hasCourseInstance'

This is an issue for the Schema.org Metatag module, rather than Metatag itself.

I was able to work around this by adding a new hasCourseInstance field to the Course type and then populating in an implementation of hook_metatags_alter(). My solution adds the 'hasCourseInstance' field, and via the module UI you can add a 'CourseInstance' to the field. But I haven't yet been able to figure out how to add CourseInstance specific fields to the type. Right now it just uses those defined by the Event type.

Do I need to create a new PropertyType of CourseInstance that duplicates the existing Event type and adds the custom fields? Or is there a better way to handle the CourseInstance specific fields like 'courseMode' and 'instructor'?

Here's the plugin code I used for the hasCourseInstance field:

namespace Drupal\dme\Plugin\metatag\Tag;

use Drupal\schema_metatag\Plugin\metatag\Tag\SchemaNameBase;

/**
 * Provides a plugin for the 'schema_course_hasCourseInstance' meta tag.
 *
 * Custom schema.org field for CourseInstance data. Can be removed when
 * https://www.drupal.org/project/schema_metatag/issues/3410151 is resolved.
 *
 * @MetatagTag(
 *   id = "schema_course_hasCourseInstance",
 *   label = @Translation("hasCourseInstance"),
 *   description = @Translation("RECOMMENDED BY GOOGLE. The course instance(s)."),
 *   name = "hasCourseInstance",
 *   group = "schema_course",
 *   weight = "0",
 *   type = "string",
 *   secure = FALSE,
 *   multiple = TRUE,
 *   property_type = "event",
 *   tree_parent = {
 *     "CourseInstance"
 *   },
 *   tree_depth = 0,
 * )
 */
class SchemaCourseHasCourseInstance extends SchemaNameBase {

}

And the code I used to fill it in with the custom fields:

/**
 * Implements hook_metatags_alter().
 */
function dme_metatags_alter(array &$metatags, array &$context) {
      $courseInstance = [
        '@type' => 'CourseInstance',
        'courseMode' => 'online',
      ];
      $metatags['schema_course_hasCourseInstance'] = serialize($courseInstance);
  }
}

In case this is helpful for anyone else.