The watermark object is an instance accessible from the main mupdfwv.MuPDFWebViewer instance as follows: 
const watermark = mupdf.watermark;
This assumes you have returned your instance name as mupdf from the initMuPDFWebViewer() promise!

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watermark

The watermark object has the following methods:

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create

create(config: { watermarks: (mupdfwv.TextWatermark | mupdfwv.ImageWatermark)[] })
  • Arguments:
    • config – An object containing an array of text or image watermarks.
Config object:
  • Arguments:
    • watermarks – (required) Array of text or image watermarks.