This text is the watermark I want to add. Just wondering how to change its transparency.
https://maketecheasier.com/watermark-pdf-pages-quick-actions-macos has a solution with automator to achieve the text transparency adjustment.
And I found another solution, using Keynote:
Load Image in Keynote, and text in Keynote can set transparency using View menu -> Show colours (CMD + Shift + C).