When signing into Survey123 Connect via a Portal named user, you are prompted to provide an API url. Is it possible to host the API internally and expose this to the client? The environment is in a disconnected state and has no internet access. I have seen mention of this some years ago but cannot source any recent documentation or mention of this functionality.
Currently, the Survey123 API is not available for an on-premises installation. As it happens, we are actively working on this and would be happy to follow up with you or your customers if they are interested in beta-testing this functionality.
We had to step back from the deployment that was under development at the time of this thread. We are beginning to document a new deployment process and will be posting it into Survey123's Esri Early Adopter Community when ready for testing (which will probably be a bit after the Developer's Summit).
