_message_form.html.erb

<%= form_tag({controller: "private/conversations", action: "create"},
              method: "post",
              remote: true) do %>
  <%= hidden_field_tag(:post_id, @post.id)  %>
  <%= text_area_tag(:message_body,
                    nil,
                    rows: 3,
                    class: 'form-control', 
                    placeholder: 'Send a messsage to the user') %>
  <%= submit_tag('Send a message', class: 'btn send-message-to-user') %>
<% end %>