You can find your collection code by going to the Service Provider Partner Center and following these steps:
Sign in to your Partner Center account.
Select your client's account.
Go to Book Catalog.
Click the Advanced button and select Manage templates.
Look for the 7-digit collection code next to the template name. (If you can not find it, refresh the page with F5 button.)
Now you can copy this code to the Google process in PublishDrive’s platform when you entrust us to handle your Google Play Books account.
Source: Google Books Help