It's really simple to automatically add Skype for Business web conferencing to meetings scheduled with FreeBusy:
Go to Calendars and Integrations.
Click on Integrations tab and Link Another Integration.
Find Skype for Business and click Add.
Sign in with your Office 365 credentials and your Skype for Business web conferencing provider will be automatically linked to your FreeBusy account.
Then refer to this guide on how to select your default conferencing provider for meetings scheduled with FreeBusy.