Displaying a Field Based on Another Field Selection

In Atlassian's support JIRA, when a user creates an issue with "Critical" priority, it will display the "Priority Explanation" field.

This can be achieved by performing the following steps:

1. Create a "Free Text Field (unlimited text)" custom field type (Administration -> Issue Fields -> Custom Fields)
2. Fill in the following text into the "Description" field:

   <script type="text/javascript">
     priority = document.getElementById('priority');
     if (priority) {
         target = document.getElementById('customfield_10000');
         // Hide the target field if priority isn't critical
         if (priority.value != 2) target.style.display='none';
    
         priority.onchange=function() {
             if (this.value == 2) {              
                        target.style.display = '';  
                        target.value="enter message here";
                     } else {
                    target.style.display='none';
             }
         }
     }
    </script>
   

3. Make sure to change the custom field id and priority id. To find the custom field id, view the source of the page when viewing an issue, or check the URL when editing a custom field.