Launch the Google Play app on your Android device. The top banner will have a “gift offer” for $ 1 of Google Play credit. The credit must be used by September 24, 2017.

As near as I can tell this must be done through the app; it is not available on the Google Play web site.

EDIT: Appears this may be YMMV. Has shown for everyone I’ve talked with here in person and on various Discord channels, but others on FW report seeing nothing of the sort.