根据您的代码，希望您已经注册了CPT（自定义帖子类型），并提供了有效的标签和；具有函数的参数：

register_post_type( \'freebie\', $args );

// Adding meta box for freebie custom post type
function demo_url_meta_box() {

    add_meta_box(
        \'demo_url_meta_box\',
        __( \'Demo URL\', \'\' ),
        \'demo_url_meta_box_callback\',
        \'freebie\',
        \'side\',
        \'low\'

    );
}

add_action( \'add_meta_boxes_freebie\', \'demo_url_meta_box\' );
// Callback function of metabox
function demo_url_meta_box_callback( $post ) {

    // Add a nonce field so we can check for it later.
    wp_nonce_field( \'demo_url_nonce\', \'demo_url_nonce\' );

    // postmeta key: _demo_url
    $demo_url = get_post_meta( $post->ID, \'_demo_url\', true );


    echo \'<textarea style="width:100%" id="freebie-demo" name="freebie-demo">\' . esc_attr( $demo_url ) . \'</textarea>\';
}
/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id
 */
function save_demo_url_meta_box_data( $post_id ) {

    // Check if our nonce is set.
    if ( ! isset( $_POST[\'demo_url_nonce\'] ) ) {
        return;
    }

    // Verify that the nonce is valid.
    if ( ! wp_verify_nonce( $_POST[\'demo_url_nonce\'], \'demo_url_nonce\' ) ) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we don\'t want to do anything.
    if ( defined( \'DOING_AUTOSAVE\' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Check the user\'s permissions.
    if ( isset( $_POST[\'post_type\'] ) && \'page\' == $_POST[\'post_type\'] ) {

        if ( ! current_user_can( \'edit_page\', $post_id ) ) {
            return;
        }

    }
    else {

        if ( ! current_user_can( \'edit_post\', $post_id ) ) {
            return;
        }
    }

    /* OK, it\'s safe for us to save the data now. */

    // Make sure that it is set.
    if ( ! isset( $_POST[\'freebie-demo\'] ) ) {
        return;
    }

    // Sanitize user input.
    $de_url = sanitize_text_field( $_POST[\'freebie-demo\'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, \'_demo_url\', $de_url );
}
// add meta box data when save_post is hooked
add_action( \'save_post\', \'save_demo_url_meta_box_data\' );