How to keep video same size as anchored image target(Poster)

I'm using ARCore and Unity. For AR doing the poster example I find the target image, get anchor and replace the poster with a video that brings the poster to life. How do I make sure the video is always the same size as the poster. I found apply ratio fitter but have no idea were to add it in the script. Any help would be greatly appreciated. Thanks.

 

Last post should be "aspect ratio fitter" not "apply ratio fitter".