Microsoft Excel doesn’t come with a built-in watermark button. But you can use an image, like a logo, to create a watermark.
Create a watermark
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Select Insert > Header & Footer.
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Tap on the header and select Picture.
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Insert your image. In the header, you’ll see & Picture.
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Tap anywhere outside the header to see the watermark.
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Select Picture > Color > Washout to change the look of the image. Grayscale may also work well.
Edit the watermark
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Tap back into the header. Select Format Picture under the Design tab.
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Select OK to apply the settings.
Source: Microsoft