#1. First, use Squarespace ID Finder to find the ID of the Button Block
In my example, we will have:
#block-yui_3_17_2_1_1717399220695_40609

#2. Use this code to Custom CSS box
/* Hide button initial */
div#block-yui_3_17_2_1_1717399220695_40609 {
display: none;
}

#3. Use this code to Website Tools > Code Injection > Footer (or Page Header Code Injection)
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function () {
setTimeout(function(){
$('div#block-yui_3_17_2_1_1717399220695_40609').fadeIn(500);
}, 5000);
});
</script>
