How Do I Create a Round Video Mask In Keynote?

First thx Gary for responding to the question I posted yesterday. Much appreciated!
New question – I know I can take any photo and have it displayed / masked into a shape say a circle. What I’m trying to do know is have the same effect for a video i.e., have a video play inside keynote but instead of it playing in a rectangular shape have it played in a circle. Is this possible within Keynote?
If not, can I create a ‘transparent round cutout’ in Preview save it as a PNG and then place that in front of the video? If so how?
Thx again…
—–
Carlos Rivera