To showcase a video you embedded in your card using portrait format, add "aspectratio="9/16" in your code as exemplified below:
<iframe width="560" height="315" src="https://www.youtube.com/embed/code" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="" aspectratio="9/16"></iframe>