通常時は自動的にidが設定される
<%= form_for(@movie) do |f| %> => <form class="new_movie" id="new_movie" action="/movies" accept-charset="UTF-8" method="post"> <% end %>
任意に設定したいとき
form_for(record, options = {}, &block)
なので
<%= form_for(@movie, {html: {id: "任意のid"}}) do |f| %> => <form id="任意のid" class="new_movie" action="/movies" accept-charset="UTF-8" method="post"> <% end %>