Follow steps 1 to 5 from this Google instruction: https://cloud.google.com/service-infrastructure/docs/service-management/getting-started#api
IMPORTANT: Please make sure to equip the service account with the following permission:
- EN: Service account permissions > Service Accounts > Service Account User
- DE: Dienstkontoberechtigungen > Service Usage > Service Usage-Nutzer
After doing the setup above visit the Google Launch Play Console, go to "Settings", select "API access" in the sidebar, scroll down to "Service Accounts" and click the "GRANT ACCESS"-Button next to the newly created Service Account.