You are prompted for authentication when you try to add an RSS feed in Outlook 2016

When Office 365 Modern Authentication is enabled, and you try to add an RSS feed that does not require authentication in Microsoft Outlook 2016, you are prompted to authenticate.

When Office 365 Modern Authentication is enabled, a header is added that can cause some RSS servers to prompt for authentication. To fix this issue, install the November 7, 2017, update for Outlook 2016 (KB4011240), and then set the following registry key to prevent the header from being placed on HTTP requests.

Important
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.

1. Exit all Microsoft Office applications.
2. Start Registry Editor by using the appropriate method for your version of Windows:
    
   • In Windows 10, go to Start, enter regedit in the Search box, and then select regedit.exe in the search results.
   • In Windows 8 or Windows 8.1, move your mouse to the upper-right corner, select Search, enter regedit in the search text box, and then select regedit.exe in the search results.
   • In Windows 7, select Start, enter regedit in the Start Search box, and then select regedit.exe in the search results.
3. Locate and then select the following registry subkey:
    

   HKEY_CURRENT_USER\Software\Polices\Microsoft\Office\16.0\Outlook\Options\RSS

4. On the Edit menu, point to New, and then select DWORD Value.
5. Enter NoAuthCheckAddingFeeds, and then press Enter.
6. In the Details pane, press and hold (or right-click) NoAuthCheckAddingFeeds, and then select Modify.
7. In the Value data box, enter 1, and then select OK.
8. Exit Registry Editor.