Setting the default value for a Rich Text Field in SharePoint Online

To set the default value of a Multiple Lines of Text aka Rich Text Field aka SPFieldNote you can use the following JavaScript:

<script type="text/javascript">
$(document).ready(function () {

var defaultText = "Your html here";

                $('nobr:contains("[Your Field Title]")').closest('tr').find('div.ms-rtestate-write').html(defaultText);

});
</script>