You will find the business rule designer within the Common Data Service area on powerapps.com. When you view an entity, look for the Business rules tab.
If your business requirement can't be achieved using a business rule, you will find that client-scripting using the client API object model provides a powerful way to extend the behavior of the application and enable automation in the client.
Use client scripting in model-driven apps
Forms in Model-driven apps help display data to the user. A form in Model-driven apps can contain items such as fields, a quick form, or a grid. An event occurs in Model-driven apps forms whenever:
- A form loads
- Data is changed in a field or an item within the form
- Data is saved in a form
Model-driven apps provides you a rich set of client APIs to interact with form objects and events to control what and when to display on a form.
Some client APIs are deprecated in the current release of Model-driven apps. Ensure that you are aware of these APIs as you write your client-side code for Model-driven apps. More information: Deprecated client APIs