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.
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
We're bringing product updates and pro tips on teamwork to ten cities around the world.Save your spot