Fluent Reader - How to use tor proxy

In the fluent reader description it mentions the following:

strict content security policies (CSP) and proxy support, meaning you can use it over Tor.

However I have been unable to find any such setting in Fluent Reader that allows for Tor support. Instead of proxy settings the app asks for the url of a PAC address. This address field rejects localhost:9051 as invalid.

Does anybody know to make Fluent Reader work through tor as the app description suggests?