require 'slack-ruby-client'

Slack.configure do |config|
  config.token = ENV['SLACK_API_TOKEN']
  fail 'Missing ENV[SLACK_API_TOKEN]!' unless config.token
end

client = Slack::Web::Client.new

client.auth_test

general_channel = client.channels_list['channels'].detect { |c| c['name'] == 't3' }

client.chat_postMessage(channel: general_channel['id'], text: 'Hello World', as_user: true)