When setting up a meeting with Outlook (either client or Outlook Web Access [aka OWA, aka Outlook on the Web]), it is quite easy to make it an online meeting (either Skype for Business or Teams).
The problem when using Outlook Web Access is you can not define the ‘online meeting’ as the default option when creating a meeting; well, not anymore
You can now set the default option for a meeting to be online by accessing your mailbox using Outlook Web Access and then go the Settings (Settings menu [gear button] and then View all Outlook settings)
Then you go to the Calendar section and Events and invitations to turn on the Add online meeting to all meetings option
When you turn this on, this is also going to be available/applicable to meetings you create when using Outlook Mobile on your iOS or Android device.
You can also manage the option from your Outlook Mobile application by accessing the Settings menu and your mailbox account settings
Exchange Administrator Actions
As this is a end-user action, Exchange administrators may want to set the option automatically (either on or off) for all users.
This is quite easy by connecting to Exchange Online using PowerShell (recommendation, use the Exchange Online PowerShell module v2 available here https://www.powershellgallery.com/packages/ExchangeOnlineManagement/1.0.1).
To manage this option for everyone, run the following commands to connect to Exchange Online and then turn on/off the settings
You can already run it now even if your end-users do not have yet the option to manage this option
Set-OrganizationConfig -OnlineMeetingsByDefaultEnabled $True (or $False if you want to turn it off)