Appetize.io supports specifying a proxy server to route traffic, or you can use a tunneling tool like ngrok.io.
If you'd like the Appetize.io traffic to go through a proxy server, you can specify the proxy in a query parameter. More info at https://docs.appetize.io/
If you want your localhost available from Appetize.io, then you can also use a tool like ngrok.io to tunnel to your localhost. More info at http://ngrok.com.