How to run discourse on subdirectory of external domain?

I have tried following that guide…

 DISCOURSE_RELATIVE_URL_ROOT: /blog
 DISCOURSE_HOSTNAME: forum.ultraluz.com.br

So I expected to be able to access the forum going to forum.ultraluz.com.br/blog but I only get connection refused so that’s what I think it’s confusing on that guide.

Is not clear what URL we should use to be able to fetch assets…