Issues & questions about react app scaffold

I'm trying to build a Zendesk Support App. It seems that some of the react documentation is mismatched between the wiki and the github repository, so some things are unclear.

Mainly, I'm having trouble setting the ticket sidebar url here.

"location": {
    "support": {
      "ticket_sidebar": {
        "url": "http://localhost:3000/",
        "flexible": true
      }
    }
  },

This default URL works when testing locally, but when installing the app in zendesk, it just displays “localhost refused to connect”. The app works perfectly when testing locally, however. I've tried changin the url to “assets/index.html” as seen in other posts to no avail. Is there a specific location where I should point ticket sidebar to for production that I'm not seeing?