Configure Default Retention Message
Set the default retention message the App Store shows for one subscription product in one locale, replacing any default configured before. Only text-based messages, with or without an image, can be defaults, and both the message and its image must be APPROVED. Products without a default in a locale show no retention message there, and the default is also the fallback when your real-time endpoint fails. Apple grants Retention Messaging API access per developer account; without it these endpoints answer 404 with no error body.
app_store_server.configure_default_retention_message