Following the documentation:
Create a microflow which receives your 'credentials' entity and passes it as a parameter to the 'Google Authenticator Connector - Create user credentials' action. The action only changes the 'ga_SecretKey' attribute in your entity so you need to commit the entity yourself. (This way you're free in how you take care of your entities and storing of objects).
The microflow will generate the secret key and sets the ga_SecretKey attribute.