how to add the “last active” feature to my profile?

So basically what you're wanting to do is hide the online indicator on your profile? That can be done with a code like this added in your CSS

/* ===== FORCE ONLINE STATUS VISIBLE ===== */
.online {
display: inline-block !important;
visibility: visible !important;
opacity: 1 !important;
position: relative;
z-index: 10;
}

Edit: saw you wanted it visibility instead

Edit 2: before placing the code, try to Ctrl+F of Find in page and look for .online and erase any duplicate old .online structure before adding this one into your CSS to avoid conflict

thanks https://spacehey.com/profile?id=4440048 !

If i understand correctly, your "last active" is already there automatically, if previous code did not hide it. You can't see it while you're logged in because you're online, you can only see it on your own page while logged out.