This plugin shows how to protect a document using a watermark.
It is without interface plugin and isn't installed by default in cloud, self-hosted and desktop version of ONLYOFFICE editors.
- In plugin code in "initSettings" field the document settings.
- Reopen a document.
If you need more information about how to use or write your own plugin, please see this https://api.onlyoffice.com/docs/plugin-and-macros/get-started/