The process is pretty simple in general. However, as I discovered, for the first time users unfamiliar with the nuances of Enterprise GIS versus ArcGIS Online, the initial process could be slightly overwhelming.
Download Survey123 application from the App store (Android or IOS). Click to open for the first time.
Choose Manage ArcGIS connections
Add your Portal address address.
Go back to the Home page. Portal connection would replace Connect to ArcGIS Online. Click on the Portal connection and proceed to sign in according with your company’s policies.