form_tag works, but I think `form_for` makes things easier. In the concerts show controller, instantiate a new `concert_song` object and assign it to an instance variable. Then you can simply write:
```ruby
<%= form_form @concert_song do |f| %>
  <%= f.hidden_field :artist_id, value: @artist.id %> # if you have access to an artist instance
  <%= hidden_field_tage :concert_id, @concert.id %>

  <%= f.select :song_name, @concert.songs.map { |s| s.title } %>

  <%= f.submit "Add Song" %>

<% end %>
```