You need to use an oval mask to remove the corners that you do not need.
The mask can be created in any image editing program - if you use a black/white mask then you need to apply Chroma Key, Alpha to it when on the timeline and set the direction arrow to get the correct display.
In the example below I used a PNG file with a clear oval with a hard edge.
on the timeline as shown below
Note - using a PNG file with a clear oval does not require use of Chroma Key effects, and also make the mask at least the same resolution as the project or higher - I make mine at double the resolution.
I can't get this to work I'm using MMEP 2015. I'm in the Chroma Key section under Video effects, I select load an effects mask and this gives me two choices SD or HD Masks, Iv'e tried both with an oval mask white oval and also a black ovel. In your picture you still have the black outline around the the oval I want to get rid of that as well.
. . . . In your picture you still have the black outline around the the oval I want to get rid of that as well. . . .
Exporting a video with transparency in the black area is not possible from MEP. you have to have something ie a background colour, image or another video clip.
Rather than give multiple possibilities / examples - what do you want as a replacement for the black area?