How to implement phx-track-static

I tried to implement the phx-track-static functionality found in the documentation, but it didn’t work as expected
it is configured exactly as in the documentation. And that is the way out. It indicates that it recognized my two files

[info] CONNECTED TO Phoenix.LiveView.Socket in 138µs
  Transport: :websocket
  Serializer: Phoenix.Socket.V2.JSONSerializer
  Parameters: %{"_cache_static_manifest_hash" => "PHOENIX_CACHE_STATIC_MANIFEST_HASH", "_csrf_token" => "PHQ2FhsGBS1wH1wjAwoZF0EdTTpXCVUYUCismkdUBK7daPUgyz4e987i", "_mounts" => "0",

 "_track_static" => %{"0" => "http://localhost:4000/css/app.css", "1" => "http://localhost:4000/js/app.js"}, "vsn" => "2.0.0"}

How can I run in a dev environment?