I have tried to achieve this by adding a trigger to a transition in my workflow and giving it a post function for updating an issue field. The problem is that the "Fix Versions" field is not available for updating in either the "Update Issue Field" or the "Update Custom Issue Field" drop down boxes. I have tried to associate the transition with different screens that do or do not contain the "Fix Versions" field to try and see if it would change the fields available to update but I've had no luck. I'm confused why the "Update Issue Field" only has a small limited amount of fields from total issue field list.
Any help would be appreciated!
