How do I enable prefetching on a cache miss

As mentioned in the Moovweb Developer Docs,

By default, the XDN will only serve prefetch requests from the edge cache. If a request cannot be served from the cache, a 412 status is returned. This protects your origin servers from additional traffic associated with prefetching.

How do I enable serving all prefetch requests regardless of whether or not the request can be served from the edge so that I can improve front end performance and warm my cache?

You can use the includeCacheMisses option when you install the prefetcher:
https://docs.layer0.co/docs/api/prefetch/modules/window_install.html#defaults.includecachemisses

ex.

 install({
    includeCacheMisses: true
  })

Please be aware that this will put additional strain on your API/origin servers and have proper monitoring in place on those servers before changing this option.