Click the WordArt icon on the drawing toolbar.

Choose a style of WordArt. Probably the 2nd one in the top row would be a
good choice. Hit OK.

Type the word Confidential. Hit OK.

In the WordArt toolbar, click "recolor."

In the Format WordArt dialog, change the fill color to light gray. Click OK.

Size the word by dragging the white size handles at the corners. Drag the
yellow diamond to change the slant if you wish.

Make sure the slide thumbnail pane to the left of the slide workspace is
showing. If it's not, hit View|Normal (Restore Panes).

Ctrl+C to copy the WordArt, then View|Master|Slide Master to go to slide
master view.

Ctrl+V to paste the WordArt on your slide master. Do the same on any title
masters and any other slide masters as appropriate.

View|Normal and delete the original WordArt from the slide.

A slight twist on Echo's method which is harder to remove

Start with a blank layout slide with the chosen background. Insert the word
art as Echo says

Save as > save as type jpeg - current slide only
On the master change the background to the jpeg you just made