how to add html id to rails form_tag

For <element>_tag you specify HTML attributes directly, as follows:

<% form_tag session_path, :id => 'elm_id', :class => 'elm_class',
                          :style => 'elm_style' do -%>
   ...
<% end -%>

It is for <element>_remote_tag constructs that you need to move the HTML attributes inside a :html map:

<% form_tag form_remote_tag :url => session_path, :html => {
                            :id => 'elm_id', :class => 'elm_class',
                            :style => 'elm_style' } do -%>
   ...
<% end -%>