Linda,
You are exposing a webservice, which will allow external systems to send info to your application. Inside the code of the exposed action/function, you'll need to store the information in your OutSystems database, or launch some notification system (or both) depending on your needs.
The most simple solution is that you store the info in a database table and have a screen or a webblock inside your application that shows "unhandled records" from that table. In this easy setup, the latest information will only be visible if the user refreshes the screen. (or you force the screen to auto-refresh at a certain interval)
If you really need to actively "push" a notification to the user, you'll have to integrate a push notification provider, or work with webhooks or something similar.
Does 1 of these solutions match your requirements?