Exclude (or include) events based on custom field value

I would like to be able to exclude or include events based on the value of a custom field dat I created. There is no option to pass it as an argument to the shortcode [events], right?

If I could do that I could use something like this:

add_filter( 'event_manager_get_listings', 'filter_by_ongoing_field_query_args', 10, 2 );
function filter_by_ongoing_field_query_args( $query_args, $args ) {

    if ($args->custom_field === "somevalue") :
              $query_args['meta_query'][] = array(
                          'key'     => '_custom_field',
                          'value'   => 'somevalue',
                          'compare'=>'NOT EXISTS',
                          );
            endif;
    return $query_args;
}

Does anyone have ideas how to pass the variable so I can filter the events listing?