I went to find certain community questions about add placeholder text or hint text into custom field, and found that the answer is to use JavaScript like:
<script type="text/javascript">
document.getElementsByName('customfield_10700')[0].placeholder='This will appear as a placeholder';
</script>
place into custom field's description, but it didn't work at all like below:

It displayed at below the field instead of display inside the field.
Any suggest solutions to add placeholder/hint text into field?