Install this release for now. Ignore any security risk warning. In Windows 11, should that pop up, click the caret to the right of the message and ALLOW.
Crop your subject, choose 1:1 in the drop-down box, position the result by dragging the image in the Preview window. Export at Letterbox setting.
Your query was answered in the Reddit forum, with a video showing the result. As it's moved, the cropped area will sweep the video. The checkerboard indicates where transparency will be. If this isn't what you had in mind try the Zoom effect.