Add a watermark

  1. Select View > Slide Master.

  2. Scroll to the top of the thumbnail pane and select the Slide Master at the top.

  3. Select Insert > Text Box, then drag to draw a text box on the slide.

  4. Type the text you want in the text box.

Change the look of the watermark text

  1. Click and hold the rotation handle at the top of the text box by moving the mouse left or right.

  2. Select the text in the text box.

  3. Choose a light font fill color, and make any other changes to the font and style.

  4. Close Slide Master. All slides in your presentation will have the watermark text.

Source: Microsoft