<%= simple_form_for blog,
  html: { class: 'form-horizontal' },
  wrapper: :horizontal_form do |f| %>
    <%= f.input :subdomain, :wrapper => :append do %>
      <%= f.input_field :subdomain, :class => 'form-control' %>
      <span class="input-group-addon">.mdwriter.io</span>
    <% end %>
    <%= f.input :title %>
    <%= f.input :subtitle %>
    <%= f.input :description %>
    <%= f.submit :class => 'btn btn-primary pull-right' %>
  <% end %>

<%= simple_form_for blog,
  html: { class: 'form-horizontal' },
  wrapper: :horizontal_form do |f| %>
    <%= f.input :subdomain, :wrapper => :append do %>
      <%= f.input_field :subdomain, :class => 'form-control' %>
      <%= content_tag :span, ".#{Rails.application.secrets.domain_name}", :class => 'input-group-addon' %>
    <% end %>
    <%= f.input :title %>
    <%= f.input :subtitle %>
    <%= f.input :description %>
    <%= f.submit :class => 'btn btn-primary pull-right' %>
  <% end %>

Modify subdomain input read config domain name.
Modify subdomain input read config domain name.
