You need to have something like this in you endpoint config:

http: [
    dispatch: [
      {:_, [
        {"/raw_socket", MyAppWeb.RawSocket, []},
        {"/phoenix/live_reload/socket/websocket", Phoenix.Endpoint.CowboyWebSocket,
          {Phoenix.Transports.WebSocket, {MyAppWeb.Endpoint, Phoenix.LiveReloader.Socket, :websocket}}
        },
        {:_, Plug.Adapters.Cowboy.Handler, {MyAppWeb.Endpoint, []}}
      ]}
    ]

Notes:

• you should only include the second entry (live reload socket route) in :dev environment
• you might need to include https config as well/instead

Now, you can use the standard cowboy handler, for example:

defmodule CustomDispatchWeb.RawSocket do
  @behaviour :cowboy_websocket_handler

  def init({_transport, :http}, _req, _opts), do:
    {:upgrade, :protocol, :cowboy_websocket}

  def websocket_init(_transport, req, _opts), do:
    {:ok, req, nil}

  def websocket_handle(_msg, req, state), do:
    {:ok, req, state}

  def websocket_info(_msg, req, state), do:
    {:ok, req, state}

  def websocket_terminate(_reason, _req, _state), do:
    :ok
end