Schema plugins are backend extensions that allow you to add fields to an APIs object. Once added with a schema plugin, fields can be read and written using the object's regular endpoints, just like any other field in the object.
To add a schema plugin extension to your app:
Select an app from the Custom Apps page in your Wix Studio workspace.
Select Extensions in the left side-menu, and click + Create Extension.

Search for schema plugin in the search bar. Choose the object you want to extend and click Create.
If you haven't set an app namespace yet, a popup prompts you to choose a namespace. You can't change your app namespace after you set it.

Use the JSON Editor to define your schema plugin as a JSON Schema. The editor's linter indicates if there are any errors in the JSON Schema.

Once your schema plugin is defined, click Save.
Important: Once you add a schema plugin to an app, it can't be removed. You can archive the plugin fields to prevent them from being read or written.
Your app now has a schema plugin extension. You can return to the JSON editor to make non-breaking changes to the schema.