Hello, I'm adding a new menu item in the backend,

Spree::Backend::Config.configure do |config|
  # ...
  config.menu_items << config.class::MenuItem.new(
    [:rejected_orders],
    'ban',
    url: '/admin/orders?q[rejected_eq]=true',
    position: 0
  )
end

How only show the menu item after admin login like the others ?

You can add a

condition

argument, where you can play with authorization to determine who can see that. As it is done here for example: https://github.com/solidusio/solidus/blob/3193519d79fdca44b585872ac6368b5e486eb39d/backend/lib/spree/backend_configuration.rb#L107

Nice ! Thank you ;)