The what3words app widget is only available for iOS devices.
To add the widget to your device, press and hold the what3words app, tap 'Add widget', then swipe left to find it.
Alternatively, you can swipe right on your home screen until you reach the page with a search bar at the top. Next, scroll down until you see an ‘Edit’ button. Tap the button and scroll down the 'Add Widgets' page until you see what3words. Tap the green cross, and tap 'Done'