Hello,
I inserted a link in the alert messages of the "Essential" theme by modifying the "frontpage.php" file found in "moodle / theme / essential / layout / frontpage.php"
more exactly, I added the link for the first alert as follows (the one I put in bold):
<section>
<!-- Start Main Regions -->
<!-- Start Alerts -->
<!-- Alert #1 -->
<?php if ($enable1alert) { ?>
<div class="useralerts alert alert-<?php echo \theme_essential\toolbox::get_setting('alert1type'); ?>">
<button type="button" class="close" data-dismiss="alert"><span
class="fa fa-times-circle"
aria-hidden="true"></span></button>
<?php
$alert1icon = 'alert' . \theme_essential\toolbox::get_setting('alert1type');
echo $$alert1icon.'<span class="title">'.\theme_essential\toolbox::get_setting('alert1title', true);
echo '</span>'.\theme_essential\toolbox::get_setting('alert1text', true); ?>
<a href="https://yoursitepage" target=_blank> link </a>
</div>