I found this snippet code here and it works fabulous! https://codepen.io/Repkit/pen/PzJqZe
Just add the HTML as an embed block where you want your video to appear:
<div class="container">
<iframe class="media" width="560" height="315" src="https://www.youtube.com/embed/rk16JND1kOY" frameborder="0" allowfullscreen></iframe>
</div>
and then this part goes in the custom CSS
#block-id-here .media{
-webkit-box-shadow: 10px 10px 61px 6px rgba(0,0,0,0.75);
-moz-box-shadow: 10px 10px 61px 6px rgba(0,0,0,0.75);
box-shadow: 10px 10px 61px 6px rgba(0,0,0,0.75);
}
#block-id-here .container:before {
content: '';
z-index: 2;
position: absolute;
width: 560px;
height: 315px;
-webkit-box-shadow: 0 0 0 7px rgba(0,0,0,0.75);
-moz-box-shadow: 0 0 0 7px rgba(0,0,0,0.75);
box-shadow: 0 0 0 10px rgba(0,0,0,0.75);
/* box-shadow: 0px 0px 61px 6px rgba(0,0,0,0.68); */
pointer-events: none;
-webkit-border-radius: 20px;
-moz-border-radius: 20px;
border-radius: 20px;
}
Here is the final result in a Squarespace web page: