No arguments are passed to the onSubmitSuccess callback function. If we take the example from the project's readme the value of values will be undefined. In some use cases it would be useful to get at least a result containing a firebase ref to the newly created entry.
Workaround. This has been pointed out to me by the creator, Tarki Huber, in an exchange of emails and will work for most scenarios:
Create a dummy entry with an empty .push() to firebase and then pass the key of this ref to the form's uid.