It is done using field configurations. See https://confluence.atlassian.com/adminjiracloud/specifying-field-behavior-776636443.html#Specifyingfieldbehavior-requirefield.
Edit: Just noticed that you want it mandatory only on certain transitions. In that case you have to use a workflow validator. You can use the "Fields Required" validator from JIRA Suite Utilities plugin.
You can use it during transition with the "Mandatory field validator" from Workfow Essentials. It also puts an asterisk * in front of the field.
Since launching the Jira Software 8.0 and Jira Service Desk 4.0 platform releases in February, many of you have been asking when the next Enterprise release will be so you can take advantage of the m...
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