Problem when masking

Hi I'm trying to replicate this effect in hitfilm express 2017 , but I'm not able to modify the mask like in this video, could you help me?

 

When I try to resize the mask just from the left, the right part also moves, I tried with control key, shift key and alt key but they don't work.

I'm not able to select just a part of the mask and animate it, even I tried to animate just the plane without the mask but the animation doesn't work, How can I do that effect in hitfilm express 2017?

 

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