If you are wondering why the Lync or Skype for Business sign-in address is not auto-populated for Lync Online users signing in on a domain joined machine, chances are that either of the following 2 Active Directory attributes are not populated:
To edit AD attributes from AD Users and Computers, first make sure you have enabled “Advanced Features”:
If you have deployed the Active Directory schema extensions required for Skype for Business or Lync on premises, then you can use the msRTCSIP-PrimaryUserAddress attribute. In many cases if you are using Office 365 Lync Online this will not be the case, so you will use the next option rather than extending the schema for this specific purpose.
If msRTCSIP-PrimaryUserAddress is not available to you, then you can add a sip record to the proxy addresses list in the following format: