[Events Manager] Make Custom Field Required

I have used the plugin home page tutorial to add a custom input to the events page; how can i make my custom field required

i have tried adding it to the var $required_fields which resulted in an error;
Fatal error: Cannot access empty property in D:\servers\site\website\wp-content\plugins\events-manager\classes\em-event.php on line 593

i have tried updating the required field’s values when creating/saving my custom field but nothing happens

Any other suggestions?