The instructions are already provided on the shared topic as following.
Not in place of image but you can place it in place of Widget content text by using the custom plugin shard in the following topic.
What it means that you can add video code in the Content text field of parallax widget to display it.
If you are not a developer then you can consider hiring a developer to develop it for you. You can hire a developer from any freelance site. Colorlib recommends the developer https://www.upwork.com/freelancers/~011652ffec8865c6d5