Q: When exporting (sharing) a project, how to choose the freeze frame thumbnail that the clip reverts to?

I just shared (exported) a project that is a single take clip for a friend who has uploaded it onto her website. The problem is that when the video finishes playing on her site it returns to a freeze frame that seems completely random and is not the frame that I want. How do I choose the exact frame that the video will return to as the "default thumbnail?"

Thanks,

Kevin