To create a formula, click inside the cell where you want the product to appear and go to the "Layout" tab of the Word Ribbon. Click the "Formula" icon and enter "=PRODUCT" in the "Formula" field.

You can add Formulas to Tables in Microsoft Word with relative ease. To learn how to insert Formula in Word Table, read this article.

Wrangle unruly text into easy-to-read formats using Word 2013's improved tables features.