<?php
add_filter('tribe-events-bar-filters', 'setup_tribe_bar_category', 1, 1);

function setup_tribe_bar_category($filters) {
    $terms = get_terms(TribeEvents::TAXONOMY);
    $html = '<div class="search-dropdown search-category-dropdown-container"><select name="tribe-bar-category" id="tribe-bar-category"><option></option>';
    foreach ($terms as $term) {
        $html .= '<option value="' . $term->slug . '">' . $term->name . '</option>';
    }
    $html .= '</select></div>';

    $filters['tribe-bar-category'] = array(
            'name' => 'tribe-bar-category',
            'caption' => 'Categories',
            'html' => $html
    );

    return $filters;
}

add_filter('tribe_events_pre_get_posts', 'setup_my_bar_field_in_query', 10, 1);

function setup_my_bar_field_in_query($query) {
    error_log($_REQUEST['tribe-bar-category']);
    if (!empty($_REQUEST['tribe-bar-category'])) {
        $taxquery = array(
            array(
                'taxonomy' => TribeEvents::TAXONOMY,
                'terms' => explode(',', $_REQUEST['tribe-bar-category']),
                'field' => 'slug',
            )
        );
        $query->set('tax_query', $taxquery);
    }

    return $query;
}
?>