How to set featured image from external url img

I’m generating a thumb of a video and its going to the public_html folder with this

  $  thumbfile = .$  key.".png";   $  videothumb = "/var/www/bamboostock/wp-content/uploads/2021/04/".$  file_name;   $  command = "/usr/bin/ffmpeg -i " . $  videothumb . " -f mjpeg -t 1 -r 1 -y ".$  thumbfile;   system($  command); 

Now I need this image to set as Featured, but nothing is working