iOS 13
Please note that the what3words app widget is only available for iOS devices. iOS widgets display on the widget screen (locked or unlocked) which you can access by swiping in from the left of the screen.
iOS 14
For iPhones operating on iOS 14, you can add the widget to Today View, your Home Screen, or any other screen. Tap and hold the widget until it jiggles to move it to any of your phone screens.