Outlook prompts for a password and doesn't use Modern Authentication to connect to Office 365
Microsoft OUTLOOK is one of the best application to access our emails, calendars, events, etc. on a daily basis. However, once you have a HYBRID environment in your network, your end-users will start prompting for username & password, especially if you are using a PROXY server in between.
The below link will explain to you in detail about the mentioned issue and provided a resolution to get rid of it.
There are so many articles are exist in the public domain to workaround this issue. I have realized that no one is having this much time to read so many articles to fix the annoying prompts. After a couple of testing, I finally introduced the below EXE which will help you to fix mentioned error in just a couple of seconds.
Note: I will recommend you to take a registry backup before execution, which will help you to restore your machine if failure.