Help removing black background on a WebM

I'm trying to export a WebM file to be used as a notification on my twitch channel. Problem is I can't seem to remove the remaining black background, for instance the frame is 1920x1080 but the image is only say 500x1080, the remaining black background is still visible. I tried applying a "Color Key' which didn't work out too well because not every black pixel is exactly perfect black. I increased the tolerance and still didn't work too well because there are black pixels in the image itself that are disappearing also. I also tried using the "include alpha channel" option in the video settings and there were still some "not so black" pixels around the border of image left behind. I just need the background to be removed completely and be transparent. Thank you for your time.