Simple script to list all your slack channels. Good way to get the id.

list_channels.rb

require 'rest-client'

r = RestClient.post "https://yourskinbox.slack.com/api/channels.list",
                    {
                        :token => 'API_TOKEN'
                    }
r = JSON.parse(r)
r['channels'].each do |c|
  puts "#{c['name']} - id: #{c['id']}"
end