User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings
User
HKCU\software\policies\microsoft\office\16.0\common\identity
This policy setting allows you to specify which hosts can show Basic Authentication sign-in prompts to Office apps. By default, all Basic Authentication sign-in prompts are blocked, and the user is shown a message that the sign-in method isn’t allowed. If you enable this policy setting, you need to enter the hosts by name, separating the host names with a semi-colon. For example: server1.contoso.com; server2.fabrikam.com. Warning: Allowing Basic Authentication sign-in prompts isn’t recommended because it’s a security risk. Basic Authentication sign-in prompts from all other hosts will be blocked and the user will be shown a message that the sign-in method isn’t allowed. If you disable or don’t configure this policy setting, all Basic Authentication sign-in prompts will be blocked, and the user will be shown a message that the sign-in method isn’t allowed. Note: This policy setting only applies to subscription versions of Office, such as Microsoft 365 Apps for enterprise, and to subscription versions of Project and Visio. For more information, see https://go.microsoft.com/fwlink/p/?linkid=2199001.