To make site title underline on hover on Squarespace, you can use below code.
#1. All Pages
Use this code to Custom CSS box
a#site-title:hover {
text-decoration: underline;
}
#2. Homepage only
Use this CSS code
body.homepage a#site-title:hover {
text-decoration: underline;
}
#3. Other pages only, not the Homepage
Use this CSS code
body:not(.homepage) a#site-title:hover {
text-decoration: underline;
}