Maurizio, thank you for your code!
I’ve found a way without setTimeout:
<script>
document.onreadystatechange = () => {
if (document.readyState === "complete") {
document.querySelector('.gh-navigation-actions > button').addEventListener('click', function() {
const iframe = document.getElementById('sodo-search-root').querySelector('iframe');
iframe.onload = function() {
iframe.contentWindow.document.querySelector('input').placeholder = 'New placeholder text';
};
});
}
};
</script>