How to copy value in a single select field to a multi-select field using ScriptRunner?

Question

Existing field: User (single-select)

New field: Users (multi-select)

Tom Lister_fo1j4e · Answer

I agree with @PD Sheehan

There should be no problems copying a single select value to multi select field. The other way round is more difficult!

PD Sheehan · Answer

Can't you just use the built-in script: https://scriptrunner.adaptavist.com/5.5.7/jira/builtin-scripts.html#_copy_custom_field_values