Can I Add Watermark contains "user information" when Files are Opened or Downloaded in SharePoint 2019 on-premises. I need the best solution for this and if anyone use third-party tool to achieve this requirement. Please Advise
You could add the related column as a label in information management policy to document content type, when you open, download or print the file using this content type, the label will exist.
For detailed information, please refer to follwoing article, this is a sample about displaying version column, you could change it to createdby column.