The below example sets a new width and height of the desired padding (128) plus the existing width (iw) and existing height (ih). The video is then moved to half that desired padding (64) on the x and y axis so this new border is visible. Lastly, the colour is set to a recognised HTML colour name (white).
ffmpeg -i input.webm -filter_complex "[0]pad=w=128+iw:h=128+ih:x=64:y=64:color=white" output.webm