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1. You would have to create a Notification entity in domain model of your app module.
2. You can associate the Notification to the Account entity (or User entity if you have generalized the Account entity) with one-many type.
3. Add a create activity for the same in the Administrator's submit Microflow (Custom Action button triggering it).
4. S etting the association created in point 2 in the above create activity based on you requirement. (Single User/Account or multiple retrieved or passed in the submit MF somehow).
5. There are ways to display the Notification objects created in the User page using Listview to create a notification panel and Custom widgets like Counter to display the number of Notifications over an icon using a bit of CSS.
I hope this gives you a fair idea on how to proceed. Good luck!