i am new to libreOffice
Watermark is some graphic text inserted via menu Format > Watermark. It is a Fontwork shape with some transparency added. Anchor: To character. Wrap: In background.
After inserting you can select by additional hitting the CTRL or the SHIFT key. Or you start (F5) the Navigator, look for Drawing objects, expand and double click on PowerPlusWaterMarkObject.
After selecting you can handle this watermark text as a regular Fontwork shape.
LO Version: 6.0.4.2
Just export document as PDF at the bottom you will see watermark,type your preferred text.