You can size the recorders window to fit the video first. Hitting record with the video paused at the start point.Then, when the recorder is recording.Start the video. You can cut out the unwanted content in the editor before producing.
However, in a lot of cases.Your better off using a free application and downloading the video instead. The process is faster overall.
After spending 2 hours trying to figure it out, I accidentally found the solution when I inadvertently clicked on the inside space of the selected region. Then the youtube video started rolling. So apparently, once you've selected and captured the region, everything is frozen and you can't press 'play' anywhere, since the 'play' arrow button on the youtube video is frozen or greyed out. There is no way to actually start the video, unless you click on the area inside the selected region.
No tutorial I read or watched ever mentioned this important information. They simply gave instructions on how to set up the options for selection and capture. No tutorial mentions how to actually start playing the video you want to record.