Add an image to the canvas, and you can find a BG Remove button on the top of the canvas. (this tool works for images only)
Click it, and the image matting window will pop up and show you the result. With the Erase and Restore options, you can perfect the result in detail.
Once it is finished, click the Download button to save this picture to your computer, and then you can apply it to your video (if you close the window without downloading, the image will be lost).