This blog post from July 2019 mentions all the restrictions that come with using Survey123 from an Enterprise Portal. With Enterprise 10.8.1 Esri lifted the restriction of not being able to use attachments in the feature service. Unfortunately it's still not possible to use the feature report service from Portals with Integrated Windows Authentication. So my suggestion: either integrate the feature report service directly in the Enterprise installation of Survey123, or lift the restriction to not being able to use this service from the Portal with IWA.
