This is explained in the Guide to Testing Rails Applications in section 4 “Functional Tests for your Controllers. The get action takes a params hash and a session hash, e.g.:
get(:show, {'id' => "12"}, {'user_id' => 5})
You should be able to pass in nil for the params hash in your example, then pass in your desired session parameters:
get :index, nil, {coupon: 'abcd'}
I recommend a thorough reading of the Rails guide for anyone using RSpec for Rails testing. rspec-rails leverages the existing Rails test classes, a point which is not made very clear in the rspec-rails docs.