Add a watermark to a picture placeholder on the master in PPT 2019

If you add a picture placeholder to a slide layout in the master you can use a background consisting of a picture or text.
Right click the picture placeholder and choose Format shape. Click on the Fill option in the taskpane:

and choose Picture or texture fill, click Insert

 

Choose one of the categories, browse and insert your picture:

Now that the picture is inserted adjust the transparency using the slider.

Close the master by clicking on the Close master button.

Create a new slide using the new layout from the master.

See the example below where I inserted the company logo in the placeholder or a watermark text. I created the watermark text in PPT by inserting a rotated text box and saving it as a svg file.

It may be necessary to adjust the colour of the text or the prompt in the placeholder to secure readability.

Empty  master placeholder

Master placeholder with transparent picture

Master placeholder with transparent watermark