Application promotion ads extend your AdWords ads with a link to your application to drive downloads. Customers can choose to click on the application link instead of on your ad text headline to learn more about the application and download it.
You'll need to have an iOS or an Android app live on iTunes or on the Google Play store. You may also need your App ID (for iOS apps) or Package name (for Android apps), though you can also look up this information as you're setting up the ad.
To set up app extensions:
Go to the Campaigns tab and click on a specific campaign and ad group where you'd like to create an app ad.
Click on + New ad and choose App/digital content ad.
Select App promotion ad:
Select your App platform and enter your App ID or Package name. You can also look up your app information by name, ID, or by publisher.
Write a headline and description, and choose your destination URL.
Click on Save.
Create additional ads following...