
The Teams Meetings button in Outlook Web App will appear as part of new event creation if the user is on an early version of the new Outlook on the web.

The meeting coordinates (the Teams join link and dial-in numbers) will be added to the meeting invite after the user clicks Send. The Teams Meeting button in Outlook for Mac will appear in the Outlook for Mac ribbon if Outlook is running production build 16.24.414.0 and later and is activated with a Microsoft 365 or Office 365 client subscription. Net 4.8 installation is required to avoid a degraded experience with the Teams Meeting add-in. Users who require this add-in are advised to install Click-to-Run version of Office, as outlined in Office on Windows 10 in S mode article.

See Upgrade from Skype for Business to Teams for details. Under some circumstances, the Teams add-in is not available in Outlook.
