How do you auto select & set a custom field string from its drop down list on workflow state change

[Brettster]

I have custom field in jira cloud instance with a drop down list of text string entries.  I want to set a post event auto transition to a particular entry from the list based on workflow state change.  What do you enter to programmatically set which entry from the custom list you want to be set at transistion state.  The example is there in teh documentation for the date and time stamp setting in a field but we have a bespoke defined list of entry options of or own for this parituclar custom field and need to be able to specify an entry from this to be set on state change.  Just need a steer on how to explicitly indicate the item on our custom list to be selected on when our state change occurs! Thanks Brettster.

[Craig Nodwell]

If you're in the cloud instance this could be done with an automation.  I would suggest a scripted solution with an add-on like scriptrunner.