Are you trying to redirect an existing Discourse topic to an external URL? That’s not what permalinks are designed to do. I’m pretty sure that you can’t redirect anything under /t.
If you want people who visit https://hoidap.cheng.vn/t/cach-cua-do-nu-bao-binh-don-gian/70 to be redirected to https://cunghoangdao.info then I think your best bet is to edit the post and tell people to click the link to go to the other site.