To find the credentials, navigate to Google Cloud Platform. In the header, next to Google Cloud Platform, click the Select a project dropdown and select a project.
If you do not have Firestore set up within this project, you will be taken to the Get started screen. Click the Select Native Mode button.
Select a location from the dropdown, and then click the Create Database button.
Once you are redirected to the Firestore dashboard screen, click the Security Rules in the sidebar. Click on the Enable Firebase button, and confirm by clicking the Enable link within the modal.
Click on the hamburger menu in the top left corner. Select APIs & Services from the sidebar, and then select Credentials from the dropdown menu.
Once on the Credentials screen, scroll to the Service Accounts table. Click on the edit icon in the row with the Name as App Engine default service account.
Click on the Keys tab, and then click the Add Key button.
Select the Create new key option. Keep the Key type as JSON, and click the Create button in the modal. This will download a JSON with the needed credentials.