RPush
Use only the Unique User Identifier. This field is not accept the push notification token.
Data-only messages: When an incoming message is "data-only" (contains no notification option), both Android & iOS regard it as low priority and will prevent the application from waking (ignoring the message). To allow data-only messages to trigger the background handler, you must set the "priority" to "high" on Android, and enable the content-available flag on iOS.
The field needs to setup channelId ( stackowerflow and docs )