It seems like this functionality has been added natively to OnBase in version 17 under SCR 143974. I would recommend contacting your first line of support for more details, as it does not pertain to the Unity API.
The API can be used to create a RepeaterItem using the FormModifier.AddRepeaterItem(editableRepeater) method. You'll want to do verification in the form, or otherwise, so it only adds if no records currently exist.
