gistfile1.rb

<%= form_tag(add_category_service_path(@service), :method => 'post', :remote => true) do %>                                                                                                             
  <%= select_tag 'service-category', options_for_select(['test'] %>
  <%= submit_tag "Submit", :id => 'submit' %>
<% end %>

# controller
  def add_category
    respond_to do |format|
      format.js {
        render :text => 'fu'
      }
    end
  end

# route
add_category_service POST   /services/:id/add_category(.:format)       {:action=>"add_category", :controller=>"services"}