The start_url key is used to tell the application at what point it should start in your application in relation to your server. While we're not using it for anything as we have a single page, no route app, that might be different in a much larger application, so you might just want the start_url key to be something indicating where you want them to start off from. Another option would be to add a query string on to the end of url, such as a tracking link. An example of that would be something like this:

{
 "start_url": "/?source=AB12C"
}