what is the id of the form_for

I used form_for to generate a form, but what is the ID of this form, how
can I specify a ID for it.

<% form_for :foo, @foo, :html => { :id => 'whatever" } do |f| -%>

<% end -%>

http://api.rubyonrails.com/classes/ActionView/Helpers/FormHelper.html#M001385