You can test this example easily.
git clone https://gist.github.com/niku/7bee3d72fd1fd442e6d115330bbd963b slack_rtm_api_sandbox
cd slack_rtm_api_sandbox
mix deps.get
mix run --no-halt -e 'Supervisor.start_link([SlackRtmApiSandbox.child_spec({"YOUR_BOT_API_TOKEN", ""})], strategy: :one_for_one)'