When opening a publicly shared survey in the Survey123 for ArcGIS field app, the error message "Survey not found. Survey id...does not exist or is inaccessible" is returned if the portal URL is not added to the list of portals. Survey123 for ArcGIS is appending the item ID of the survey to the most recent portal, which in the case of public use would be ArcGIS Online, resulting in this error.