Custom Field in Page Title

Use the wp_title filter, which passes the current title, separator, and location as arguments. It then appends the custom field value to the title if the conditions are meet. For more details :- Visit Link wp_title

function wpse_224340_document_title($title, $sep, $seplocation) {
    global $post;

    // make sure the post object is available to us
    if (is_singular('mec-events')) { // check we're on a single post
        $release_author = get_post_meta($post->ID, 'mec_fields_2', true);

        if ($release_author !== "") {
            $title .= " (" . $release_author . ")";
        }
    }

    return $title;
}

add_filter('wp_title', 'wpse_224340_document_title', 10, 3);