How to capture part of an image on a video so that it can be passed to another service?

Question

Video

There are several video files in the social network or video services. There is a need to capture a part of the image from the frame at a certain moment in the video and transfer it somewhere or save it to a PC. This needs to be done not just by hand through third-party capture software, but preferably through the service API or somehow in a more or less automatic mode (if the explanation is crooked, I apologize, because I don’t understand the technical part of the word "completely ").