I use the Applied Magic Sc

I use the Applied Magic Screenplay, but this should be straight forward…
If you have a green screen in an image file, place that on the SB. Now take your subject clip and make it a picture in picture. You should now have a full green screen with the subject’s green screen sized as you wanted. Render and re-key.
If you don’t have the green screen image, create it. With the ScreenPlay, I can take a black clip and place it next to the subject that has the green screen background, I can now select any pixel in the green screen background and fill the black clip to the same color value. You should be able to duplicate this.