Out-of-the-box solution for a scroll to top or scroll to bottom button does not exist in Squarespace but you can create one easily with my little trick.

First you create a code block in any section of your page:

Inside that code block, you write an HTML code with a link tag that has an arrow character inside:

After this you can add here the css code in a style tag, where you specify the top or bottom and horizontal position of the button, the way it will look and some mouse over effect:

Finally we add a short script, with an event listener for the click event that will trigger a scroll action for the window with smooth scrolling effect. The window will scroll the entire scroll height of the document.

Ok, but how can I create a scroll to top button?
The same way as with the scroll to bottom button, but instead of the top: document.body.scrollHeight you will add a top: 0;
Also instead of ↓ html entity you can use the &uarr entity.
But I want the button to disappear when it’s on the bottom
In order to check if we arrive almost on the bottom of the document we compare the window.innerHeight + window.scrollY with document.body.scrollHeight – 20:

I will leave here the code for the codeblock for you to copy paste:
<a href="#!" id="scrollToBottom" class="scrollToBottom">↓</a>
<style>
.scrollToBottom {
position: fixed;
right: 10px;
top: 100px;
z-index: 100;
background: #000;
color: fff;
width: 50px;
height: 50px;
display: flex;
align-items: center;
justify-content: center;
font-weight: bold;
transition: opacity 0.6s;
}
.scrollToBottom:hover {
opacity: 0.85;
}
</style>
<script>
document.getElementById('scrollToBottom').addEventListener('click', () => {
window.scrollTo({
left: 0,
top: document.body.scrollHeight,
behavior: "smooth"
});
});
window.onscroll = () => {
if(window.innerHeight + window.scrollY > document.body.scrollHeight - 20 ) {
document.getElementById('scrollToBottom').style.display = 'none';
}
else {
document.getElementById('scrollToBottom').style.display = '';
}
}
</script>
Enjoy !
For a video explanation please watch my video on Youtube: