How can I make watermark text partially transparent?

We have a software PDF Stamp which can add transparent watermark text. And you can free downlaod PDF Stamp by

this link: https://www.verypdf.com/pdfstamp/pdfstamp_cmd.zip.
If you need to know more about it, you have to visit its homepage:https://www.verypdf.com/pdfstamp/index.htm.

If you use the CMD version, the command line can be inputed like the following command:
pdfstamp.exe -PDF “example.pdf” -o “opacity.pdf” -AT “VeryPDF Opacity” -FT “Arial” -P9 -FS100 -opacity50 -C#FF0000 -AI logo.jpg -opacity80 -P2 -AL -LW30 -opacity30 -LINEEX 100,100,200,200

This articel can help you know this way well.
https://www.verypdf.com/wordpress/201106/stamp-your-pdf-file-with-opacity-option-661.html