app/views/domains/_form.html.erb
<div class="box">
<h3 class="title">Add a new domain</h3>
<%= form_for [@app.server, @app, @domain], remote: true do |f| %>
<div class="control">
<%= f.text_field :name, placeholder: "Domain name", hide_label: true, class: "input" %>
</div>
<%= f.submit "Add", class: "button control is-primary" %>
<% end %>
</div>