Using HAproxy as frontend proxy

I use haproxy as a frontend of my website, and would like to serve writefreely out through it.

There are other services, so my plan was to put writefreely under e.g. and configure an haproxy frontend/backend to proxy that

frontend front_https
        acl writefreely_uri path_beg /blog
        use_backend backend_server if writefreely_uri

backend backend_server
        server             writefreely

This isn’t working. I could go through nginx proxying also or instead, but that’s messier.

Wondering if anyone has done this?