function add_custom_meta_to_content($content) {
    $queried_object = get_queried_object();

    if ( $queried_object ) {
        $post_id = $queried_object->ID;
    }
    $text = get_post_meta($post_id, 'textbox_wporg_meta_key', true);
    if( is_single() ) {
        $content = $text . '' . $content;
    }
    return $content;
}
add_filter( 'the_content','add_custom_meta_to_content' );