The biggest problem for me while working with Skype for Business [SfB] Online, has been in finding the right documentation.

As I discovered while developing a new application that needed to utilize Skype for Business Online API, that the application needs to have consent from an administrator in order to be able to authenticate the USER to use Skype for Business Online API.

Add extra permissions, other then sign-in from Azure AD in your app, i.e. Skype for Business.

Need OAuth to use with Azure Active Directory, create a Azure Active Directory application first.