#1. First, you need to find Code Block ID.
In my example, it is: #block-yui_3_17_2_1_1729047562015_20924
#2. Next, use this code to Custom CSS box
#block-yui_3_17_2_1_1729047562015_20924 {
opacity: 0;
visibility: hidden;
transition: opacity 1s ease-in-out;
animation: showElement 0s 4s forwards;
}
@keyframes showElement {
to {
opacity: 1;
visibility: visible;
}
}