Information
This policy setting controls whether network proxies are allowed to show Basic Authentication prompts.
By default all Basic Authentication sign-in prompts are blocked and the user is shown a message that the sign-in method isnt allowed.
If you enable this policy setting then network proxies will be allowed to show Basic Authentication prompts.
Warning: Allowing Basic Authentication sign-in prompts isnt recommended because its a security risk.
If you disable or dont configure this policy setting all Basic Authentication sign-in prompts from network proxes will be blocked and the user will be shown a message that the sign-in method isnt 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.
Solution
Policy Path: Microsoft Office 2016\Security Settings
Policy Setting Name: Allow Basic Authentication prompts from network proxies