Hello @Cholpon !
The first option to let your customers know about an automatic discount on a collection page specifically, is by creating an announcement for it in the description box of your Collection from within your Shopify admin, as seen below.
Adding text in this description box will display it at the top of your collection page of your storefront
The second way you can let your customers know about discounts on any pages of your store, is to use a third-party app like Quick Announcement Bar or Announcement Bar & Header Bar. These apps add a customizable banner to the top of any page you choose to add it to on your store.
Hope this helps!