Using Classic Outlook on Windows for personal email, calendar, and contact management
This feature is available in Outlook for Windows for Current Channel (Preview) users on Version 2004 (Build 12730.20150) or later. Following the registry entry will disable it.
reg add "HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Options\General" /v "EnableImprovedCaptivePortalExperience" /t REG_DWORD /d 0 /f