Applications often need a way to validate current user account. For example, completing online forms and provide signature with password. With Active Directory on-premise, the [System.DirectoryServices] namespace can be used to validate username and password with [PrincipalContext] and [ValidateCredentials].
With Azure AD, different methods are needed. Video demo below. Source code available at https://github.com/spjeff/VerifyAzureAD/
Cheers!

Video
References
- https://github.com/spjeff/VerifyAzureAD/
- http://www.cloudidentity.com/blog/2014/07/08/using-adal-net-to-authenticate-users-via-usernamepassword/
- https://vincentlauzon.com/2017/01/29/authenticating-to-azure-ad-non-interactively/
- https://stackoverflow.com/questions/38992725/get-azure-active-directory-token-with-username-and-password