1. Go to Jira Administration | Issues | Customer fields

2. Click on the Add customer field button in the top-right corner and select Field Type as Text Field (multi-line).

3. On the next screen, enter testfield as Name and enter the Javascript code into the Description field.

<script type="text/javascript">

priority=document.getElementById('priority');

if(priority){

target = document.getElementById('customfield_11502');

if (priority.value != 1)

target.style.display='none';

priority.onchange=function() {

if (this.value == 1) {

target.style.display = '';

target.value="enter message here";

} else {

target.style.display='none';

}}}

</script>