First you need a video that has a white background or black background.
I trimmed the video in the source viewer. Put it on the timeline.
Added a threshold filter. with invert checked. Adjusted the level to make the subject totally white and the background black.
Added a crop circle filter, set the radius to 35%. Added a size and position filter above the crop circle. Positioned the subject in the circle.
Exported that clip from the timeline.
Disabled the Threshold filter on the original video.
Opened the exported clip, put it on a track above the original video.
Added a Chroma Key simple filter, keyed out the white, a Blur:Low Pass filter set to 48%, a Levels filter with Input White set to 0.1, a Color Grading filter to make the background green.
Exported the video using the gif preset.