Sure, no problem. You can put the HTML code with the <span> in the After Header now, and the code below in the </head>. It displays the first name only. To show their full name, you would get rid of the .split(' ')[0].
<script type="text/discourse-plugin" version="0.8.18">
api.onPageChange(() => {
if(api.getCurrentUser() != null) {
$("#logged-in-user").text(" " + api.getCurrentUser().name.split(' ')[0]);
}
});
</script>