This is a slightly different topic. In future could you start your own new thread.
To do what you want to do you need to use a black and white mask. These can be easily made using Windows Paint as I have in the example underneath.It literally took me 30 seconds to make.
Then you can use the Chroma Key effect and if need be the size position tool to place the video you want into the bubble. You can find how to use those effects by looking for the tutorials you will find in the Media section of this site.