Had the same problem, I had to add these permissions to the app I was using to create the meeting events:
TeamSettings.Read.All, TeamSettings.ReadWrite.All
As soon as I added them, isOnlineMeeting was correctly set to true, and onlineMeetingProvider was set to teamsForBusiness, as specified in the request.
onlineMeetingUrl is always null, but it's possible to get the meeting url from the "onlineMeeting.joinUrl" property, which is automatically populated by the API.
UPDATE:
As @lokusking pointed out, even if this is correct, it looks like it does not completely solve the problem. According to my tests, creating a Team event for a new user and getting isOnlineMeeting=true and onlineMeeting properly populated in the response will work only a couple of hours after the user account is created. Note that even with a fresh user the event IS created, just those properties won't contain the right values.
