This solution assumes you already have Microsoft Teams installed.

Installing Teams plugin in Outlook

1. Quit Microsoft Teams and close Microsoft Outlook.
   1. Right click on Microsoft Teams and click Quit.
      
2. Start Microsoft Teams and sign in to it.
3. Start Microsoft Outlook.

Verifying plugin is installed

1. Open Microsoft Outlook.
2. Click on File
3. Click on Options.
4. Click on Add-Ins.
5. Ensure Microsoft Teams Meeting Add-in for Microsoft Office is under Active Application Add-ins.
   
6. If Microsoft Teams Meeting Add-in for Microsoft Office is not under the Active Application Add-ins list.
   1. Select COM Add-ins then click Go...
   2. Click on the checkbox next to Microsoft Teams Meeting Add-in for Microsoft Office then click OK.

You have now installed the Microsoft Teams plugin in Microsoft Outlook