You may either use the Outlook Connection option in the Webex App's settings (Outlook and Webex need to be on the same machine).
Better would be to use the hybrid calendar integration. That may either be enabled per org by your administrator or you can enable it at your personal meeting settings site (Settings->Meetings->Configure Advanced settings). There you can authorize Webex to access your O365 account.
We've enabled it at org level and it just works fine. Just schedule a meeting in Outlook, type @webex as location and let Webex do the rest