honkoma/docs/configuration/howto_proxy.md

How to configure upstream proxy for federation

If you want to proxify all http requests (e.g. for TOR) that pleroma makes to an upstream proxy server, edit you config file (dev.secret.exs or prod.secret.exs) and add the following:

config :pleroma, :http,
  proxy_url: "127.0.0.1:8123"

The other way to do it, for example, with Tor you would most likely add something like this:

config :pleroma, :http, proxy_url: {:socks5, :localhost, 9050}