01_with_default_scope_extension.rb

module WithDefaultScopeExtension
  def with_default_scope(scope_options, &block)
    normalized_scope_options = Array(scope_options)
    scope(*normalized_scope_options) do
      instance_exec(&block)
    end
  end
end

ActionDispatch::Routing::Mapper.send :include, WithDefaultScopeExtension

02_routes.rb

Rails.application.routes.default_scope = [':custom_prefix', custom_prefix: /app/]
Rails.application.routes.draw do
  # routes are here
end