I know this is an old thread, but I ran into the same issue so I wanted to share what I figured out. If you click the "mask sensitive data" with nothing input in the watermark area, it will force you to put that "Do Not Mail - Data Masked" watermark. However, if you put in your own text first then it will use that text.

So I just input a space and then clicked the Mask Sensitive Data button and it accepts that space as my "watermark."