Map route to a controller action.

Provide controller action body - execute csv exporter.

app/controllers/spree/admin/promotion_batches_controller.rb

@@ -7,6 +7,13 @@ def show
           format.js   { render layout: false } if request.xhr?
         end
       end
+
+      def csv_export
+        send_data PromoCodesExporter.new(params).call,
+                  filename: "promo_codes_from_batch_id_#{params[:id]}.csv",
+                  disposition: :attachment,
+                  type: "text/csv"
+      end
     end
   end
 end

config/routes.rb

@@ -13,7 +13,7 @@
 
     resources :promotion_batches do
       member do
-        get :csv_export
+        get :csv_export, to: 'promotion_batches#csv_export'
       end
     end