我最近刚刚用以下代码使用自定义的帖子类型和自定义分类法完成了这项工作。

function cpt_assign_term($post_id) {

    $current_post = get_post( $post_id );

    // Only apply to new posts.
    if ( $current_post->post_date == $current_post->post_modified ) {
        wp_set_object_terms( $post_id, \'default-term_name-or-id\', \'category\', true );
    }
}
add_action( \'save_post_{your-post-type}\', \'cpt_assign_term\' );

https://codex.wordpress.org/Function_Reference/wp_set_object_terms