Where and how I should add this code?

I want to add a code with the plugin “code snippets” that you recomend, but when I try to do it, it gives an error, could you please help me telling me where I should place it (header, foooter) and how I should write it.

This is to load the preview thumbnail of the video, and only load the video when the user clicks on the image.

This is the code:

( function() {

  var youtube = document.querySelectorAll( ".youtube" );
  
  for (var i = 0; i < youtube.length; i++) {
    
    var source = "https://img.youtube.com/vi/"+ youtube[i].dataset.embed +"/sddefault.jpg";
    
    var image = new Image();
        image.src = source;
        image.addEventListener( "load", function() {
          youtube[ i ].appendChild( image );
        }( i ) );
    
        youtube[i].addEventListener( "click", function() {

          var iframe = document.createElement( "iframe" );

              iframe.setAttribute( "frameborder", "0" );
              iframe.setAttribute( "allowfullscreen", "" );
              iframe.setAttribute( "src", "https://www.youtube.com/embed/"+ this.dataset.embed +"?rel=0&showinfo=0&autoplay=1" );

              this.innerHTML = "";
              this.appendChild( iframe );
        } );  
  };
  
} )();

I get this code from this article: https://webdesign.tutsplus.com/tutorials/how-to-lazy-load-embedded-youtube-videos–cms-26743

Thank you.