The root (base) of the site (index.html is served by default.)
Keep in mind though, any request, to any resource, (e.g. /favicon.png, /style.css. /path/to/file) will all go to this function now as force = true so the function needs to return data for everything. I’m not sure if this is what you are wanting/intending or not.
There is a previous discussion on using a serverless function to serve on the content on base URL.