When generating a Phoenix application you have code like:

defmodule Example.MixProject do
  use Mix.Project

  def project do
    [
      app: :example,
      version: "0.1.0",
      elixir: "~> 1.14",
      elixirc_paths: elixirc_paths(Mix.env()),
      start_permanent: Mix.env() == :prod,
      aliases: aliases(),
      deps: deps()
    ]
  end

  # …

  # Specifies which paths to compile per environment.
  defp elixirc_paths(:test), do: ["lib", "test/support"]
  defp elixirc_paths(_), do: ["lib"]

  # …
end

You can use exactly same way in the application function, for example:

  def application do
    [
      mod: {Core.Application, []},
      extra_applications: extra_applications(Mix.env())
    ]
  end

  defp extra_applications(:prod), do: [:logger, :runtime_tools, :google_maps, :os_mon]
  defp extra_applications(_env), do: [:logger, :runtime_tools, :google_maps, :os_mon, :wx, :observer]

Note: It’s more rare, but I guess you may want to run prod with those applications sometimes. The alternative way would be to use an environment variable when compiling or generating release.

  def application do
    [
      mod: {Core.Application, []},
      extra_applications: extra_applications(Mix.env())
    ]
  end

  defp extra_applications(:prod) do
    env = System.get_env("MY_PROJECT_USE_WX", "false")
    extra_applications(env)
  end

  # dev, test and custom envs …
  defp extra_applications(env) when is_atom(env) do
    extra_applications("true")
  end

  defp extra_applications("true"), do: [:logger, :runtime_tools, :google_maps, :os_mon, :wx, :observer]
  defp extra_applications(_env), do: [:logger, :runtime_tools, :google_maps, :os_mon]

In both ways you don’t have to change the way you generate releases, but in case you want a prod env with wx run:

$ MIX_ENV="prod" MY_PROJECT_USE_WX="true" iex -S mix phx.server
# or 
$ MY_PROJECT_USE_WX="true" iex -S mix phx.gen.release