Need to implement this page transition

Hello, Im trying to implement this code to a squarespace website so it has page transitions, i still can't do it propperly. I'm used to webflow so it's my first time not knowing the classes or how to add some divs. If someone could help me I would be so grateful

https://codepen.io/alvarotrigo/pen/LYeKGVV

Your Codepen require a lot of adjustment, so I tested it & customize this below code. The effect is quite similar your Codepen.

Add this code to Website Tools (under Not Linked) > Code Injection > Footer

<div class="splash-wrapper">
  
</div>
<style>
.splash-wrapper {
  position:fixed;
  z-index:9999;
  background-color: #000000;
  height:100vh;
  width:100vw;
  display:flex;
  flex-flow: column nowrap;
  justify-content:center;
  align-items:center;
  animation-name: slideOut;
  animation-fill-mode: backwards;
  animation-duration:.65s;
  animation-delay: 2s;
  top: 0;
  margin-left: 100vw;
}
@keyframes slideOut {
  0% {margin-left: 100vw;}
  50% {margin-left: 0vw;}
  100% {margin-left: 100vw;}
}
</style>