What should be the base.js.coffee file?

[afuno]

Its contents are by default:

#= require jquery
#= require ./jquery_ui
#= require jquery_ujs
#= require_self
#= require_tree ./lib
#= require_tree ./ext
#= require_tree ./initializers

window.ActiveAdmin = {}

And how does it need to change, to add your string?
#= require active_admin/select2