You can use something like this in your code injection footer:
<style>
.global-alert {
padding: 1.5rem 0;
background: rgba(0 0 0 / 0.15);
text-align: center;
margin-top: 3.5rem;
}
</style>
<script>
const globalAlert = document.createElement('div');
// If you need to insert markup, use this line:
globalAlert.innerHTML = '<strong>Announcement</strong>: Something to announce'
// If you only need to insert text, use this line:
globalAlert.textContent = 'We independently review everything we recommend. When you buy through our links, we may earn a commission.';
globalAlert.classList.add('gh-inner', 'global-alert');
document.getElementById('gh-head').appendChild(globalAlert);
</script>