It's really simple to automatically add RingCentral web conferencing to meetings scheduled with FreeBusy:
Go to Calendars and Integrations.
Click on Integrations tab and Link Another Integration.
Find RingCentral and click Add.
Sign in with your RingCentral credentials and your RingCentral account 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.