Watermarks in Microsoft 365

Powerpoint
For a watermark on specific slides, go to: Design Tab > Background > Image and select an image for the watermark you want.
To have the same watermark on each slide using the same layout, go to View Tab > Slide Master > Choose the slide template to apply > Format background > Picture or texture fill > insert the image you wish to use for the watermark.
Word
The online version of word does not currently support the "Design" tab where watermarks live in the desktop version. To overcome this limitation, you can place an image in the background of each page.
However, if not constructed properly the images could cause the file to be too large for editing online.