Adding to existing v3 extension for chrome feature for onscreen recording video with audio BUT not fullscreen recording but for specific selected area which user will first select on webpage.

I am looking for adding to my existing extension which is version 3 feature which will record onscreen video with audio from web page but only area which user will select. This mean NO full screen recording but only selected specific area which user will always select before user start recording. Output format can be on mp4