Using UI Extensions
Before you can use the UI extension within the content editor, you must add it to a content model. Inside the schema editor, pick your custom field from the available fields.
Next, you'll want to give it a display name, API ID, description, and any configuration, just like any other field you'd add.
Next switch to editing content, and add a new entry for the content type you added the field.
You should now see your custom UI extension rendered: