form

gistfile1.txt

<div class="row-fluid">
        <div class="services_pro col-md-6 col-md-offset-1 "> 
            <div class="well well large">
                  <p><h3>Envia un mensaje a <%= @lawyer.name %></h3></p>
                 <p><%= form_tag({controller: "messages", action: "create"}, method: :post) do %></p>
                 <%= label_tag :Asunto %>
                  <p><%= text_field_tag :subject %></p>
                  <%= label :body, "Mensaje" %>
                  <p><%= text_area_tag :body %></p>
                  <%= hidden_field_tag(:lawyer, "#{@lawyer.id}") %>
                <%= submit_tag 'Enviar Mensaje', class: "btn btn-primary" %>
                 <% end %>                 
            </div>
        </div>
    </div> 

Is this /app/views/messages/new.html.erb?

No, is not the same structure as your example, I put this code in /app/views/lawyers/message.html.erb
heres the full repo https://github.com/marcfuentes/abogado