Install VLC (https://www.videolan.org/)
Start this application, position and size the form to your needs. Hit the 'Generate VLC ViewPort' button, and a new (VLC) window will open.
The VLC Viewport can then be shared on MS Teams. Select 'Window sharing' in Teams and not 'Desktop sharing' and share the new VLC window.
Based on KeesjanVe/Desktop_Viewport idea.