Outlook on prem prompts for O365 Password

 Issue:

In an Exchange on prem environment Outlook prompts for O365 Password
trying to connect to exchange online or Office365.
and is not connecting after that.

Cause:

Moder authentication was enabled

Solution:

First, we thought of postponing the O365 request, by adding registry Dword (ExcludeExplicitO365Endpoint)
this will tell Outlook to postpone the O365 Autodiscover request to the last.
1- Open Registry Editor
2- Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Office\<x.0>\Outlook\AutoDiscover 3- 3- 3- Create new Dword : ExcludeExplicitO365Endpoint  value 1 

but this didn't solve the issue 

so we decided to disable Modern authentication

1- Open Registry Editor
2- Navigate to: HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\<x.0>\Common\Identity  
3- Create Dword:

EnableADAL value 0
DisableAADWAM  value 1
DisableADALatopWAMOverride  value 1

then the Prompt stopped and outlook connected to On-prem Autodiscover