Please bear in mind that what you're working with is just a demonstration - and only of creating a custom watermark. Watermarks always apply to an entire page and they're not the be all and end-all of using custom quick parts/building blocks.
In that case, you need to create a custom quick part/building block applicable to that; not to a watermark. You can add other reusable elements to the appropriate building block galleries by creating them then selecting the element and using Insert|Quick Parts|Save Selection to Quick Parts Gallery, along with the relevant options offered there. That said, it's not apparent why you'd want to add a textbox inside a table.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]