Update form-onsave.md

Author: mspilde

powerapps-docs/developer/model-driven-apps/clientapi/reference/events/form-onsave.md

@@ -64,7 +64,9 @@ In this scenario, since there are multiple async processes and both calls return
 
 ### Enable Async OnSave using app setting 
 
-An app setting is a platform component that allows you to override a setting on an app. 
+An app setting is a platform component that allows you to override a setting on an app.
+
+The appsetting must have a unique name and be in this format: `solutionpublisherprefix_appname_settingname` 
 
 To enable the async `OnSave` event handlers for a specific app, add the below XML in the `customization.xml`  file. This should be added in the existing AppModule node in your `customization.xml` file.