How can to add watermark in MS PowerPoint

First, we will begin by adjusting the slide master:

1. Click the View tab.
2. Select Slide master. Select the top slide.

Go to the Insert tab. Add a text box in the middle of the slide.

Format tab.

1. Now to make the text look faded and more like a watermark, RIGHT CLICK OR select 
2. Type in the text you want to be watermarked.
3. Resize, rotate, and adjust the text as you see fit.
4. Click the pop-out a menu for Format Shape. A menu will appear on the right.
5. Click the Text options icon in the format shape menu.
6. Under Text Fill and Outline, you can adjust the transparency of the text here. Adjust the text transparency as you see fit.
7. Close the master view in the top menu bar. Now, the watermark should be visible on text-heavy slides.