When using an account with Multi-factor Authentication enabled, certain applications (like Skype for Business or Outlook) require specialised logins.
To generate an App Password:
Be sure to generate a new password for each App (Skype for Business, Outlook, etc). Since passwords can't be retrieved after they've been generated, be sure to store them in a safe place.
First, login to: https://portal.office.com/account/
Choose "Manage Security & Privacy" and click the link to "Update phone numbers..."
Click, "App Passwords"
On the "app passwords" page, click the "create" button.
Give a name to the password that's generated. Creating a different password for each app is best practice.
A semi-permanent password will be created. Be sure to keep this password safe since it bypases the multi-factor authentication system. Also, once this page is closed, it's not possible to retrieve the same password. It will be necessary to generate a new app password if this one is lost.