Chroma key is a background removal compositing technique that supports removing a specific color in a video or image.

Click the Chroma Key in the toolbar above, then pick a color from the video/image, or choose from 3 preset colors.

Video:

Image:

After selecting a color, you can adjust the intensity. This value will affect the extent and range of the color being removed.

Then, how to add a new background? Easy.

First, right-click to set the video/image as a layer.

Now, just drag your desired video/image to replace the black background. Done!