Hey, Melanie !
Thanks for posting!
There is an app called Automatic Discount which can add a coupon or discount to the cart automatically.
With this app, there is also the option to display a popup or a notification which let's your customers know when this discount has been added. This can be set to appear like so. While I'm not sure if this is 100% what you're looking for, it may be worth chatting to the app developer directly to see if this pop up can be displayed elsewhere on your site too.
Hope this helps you out!
Koreen W | Shopify