How to programmatically edit user's instant notifications through the API

We are creating a process to auto-create users in Workfront when they are present in another system, but don't want the users to get all the default notifications (they are only being used for reporting at this time, and the default notifications are driving them crazy). It looks like we can make a POST call to the USERPF endpoint to overwrite the user's digest notifications and some other settings, but can't seem to update the user's instant notifications using the same approach. Any ideas?