You cannot use EndNote directly in Word Online via Teams.
However, there are some other solutions to work together via Teams.
Sharing EndNote libraries via Teams
To share references or libraries, you can:
Share the .enl file and .Data folder via OneDrive or SharePoint (but avoid editing at the same time).
Sync your library with EndNote Online and share groups with colleagues.
Export references as RIS or XML files and import them into others’ libraries.
Open Word documents in desktop Word to use EndNote features while the file is stored in Teams/OneDrive.