Set a placeholder to the Multiselect Dropdown form element

[Alison]

I want to share a solution on how to set a  placeholder to the Multiselect Dropdown form element on the search form.
You should put the following code into the Footer after disabling HTML Editor on the advanced tab:
 

<script>
document.addEventListener('DataPageReady', function () {

   document.querySelector('[id^="ComboBoxValue"]').setAttribute("placeholder", "Your value for the placeholder");

});
</script>