You can use the event_dispatcher service to trigger an event, for example with \Drupal::service('event_dispatcher')->dispatch(MyModuleEvents::MY_EVENT, new MyModuleEventClass());.
You need to replace MyModuleEvents and MyModuleEventClass with the class names you are effectively using, and MyModuleEvents::MY_EVENT with the effective event name. (It is better to use a class constant.)
You can run the code using drush php "\Drupal::service('event_dispatcher')->dispatch(MyModuleEvents::MY_EVENT, new MyModuleEventClass());".
See Subscribe to and dispatch events for more details.
