When posting a long video to Facebook that I only want to show a small portion of, how do I clip the short period I want people to see?

I have been trying to “share” a video from You Tube (and it’s also on another site) to Facebook. When I click on “embed” there’s something with “start at” and you can enter the time. However, when I enter the time I want the viewing to begin, and then I go to the actual Facebook post, the entire three hour video is up there.

How do I ensure that only the small clip I want shown will be the only part posted to FB?