Redmine/app/views/custom_fields/new.html.erb

<%= title [l(:label_custom_field_plural), custom_fields_path],
  [l(@custom_field.type_name), custom_fields_path(:tab => @custom_field.class.name)],
  l(:label_custom_field_new) %>

<%= labelled_form_for :custom_field, @custom_field, :url => custom_fields_path, :html => {:id => 'custom_field_form'} do |f| %>
<%= render :partial => 'form', :locals => { :f => f } %>
<%= hidden_field_tag 'type', @custom_field.type %>
<%= submit_tag l(:button_save) %>
<% end %>

<%= javascript_tag do %>
$('#custom_field_field_format').change(function(){
  $.ajax({
    url: '<%= new_custom_field_path(:format => 'js') %>',
    type: 'get',
    data: $('#custom_field_form').serialize()
  });
});
<% end %>