I'm using the "older" version of the proxy, not the current one on github. What I have in my proxy.config is
<?xml version="1.0" encoding="utf-8" ?>
<!-- Proxy config is used to set the ArcGIS Server services that the proxy will forward to.
mustMatch: true to only proxy to sites listed, false to proxy to any site
For Printing on a "remote" iis web machine that uses secure services on the AGS machine
a blank Proxy folder/application needs to be located and references on the remote iis machine.
The proxy folder/application for the secure services with required tokens should be located
on the ArcGIS Server web adaptor IIS machine.
mustMatch = "false" OK, since the rest of the file is basicallyblank anyway
-->
<ProxyConfig mustMatch="false">
<serverUrls>
<!-- PRODUCTION mapping -->
<!-- nothing needed here for prining -->
</serverUrls>
</ProxyConfig>
( the notes are added by me) and I actually have it in my IIS wwwroot folder in a folder I call "printProxy" for convenience. We are typically using it with JavaScript apps, and our custom print gp service, but we have it working with are ArcGIS Online printing too (the custom GP printing). For the JS apps, I point to this proxy for the printing.
I attached a copy of the proxy I'm using...not sure why I have a blank App_Data folder in this directory but I left it. It was a while ago that I set this up and didn't want to mess with it. This might not work for you but it might give you or others and different idea for a solution.