You can specify config values for the current project and its third-party dependencies at config/config.exs:

# config/config.exs
use Mix.Config

# Project config
config :mymodule,
  api_url: "http://api.seave.co"
  namespace: "hello"

# Third party dependency
config :logger,
  verbose: true

Your current project and any third party libraries will be able to pick it up through Application.get_env:

# lib/client.exs

defmodule Client do
  def log
    IO.puts Application.get_env(:mymodule, :key)
  end
end

iex> Client.log
hello