Animate a Mask in Adobe Premiere Elements

Hi Steve, thanks for responding.  I'm a subscriber to your YouTube channel and I think I've watched almost every one of your Premiere Elements Videos!  I'm trying to create the effect of an object revealing text as it moves across the screen.  Also the effect where the next scene in a video is revealed by an object as the camera pans.  I've attached two examples where I've captured screen shots.  I think the effect requires animating the mask over time - it needs to move and change shape as time progresses.  So I think the mask would need a series of key frames where at each key frame the shape and position is different.  I haven't been able to figure out how to make a mask use key frames.  Hopefully this makes sense.  If you know how to do this or figure out how to do it in Premiere Elements it would make a great addition to your library of how-to videos!  Thanks, Rick