That is the Jira built-in "update issue field" which was written for Jira 1 if memory serves (possibly Jira 2, it's been a long time). It is only capable of updating "system" fields - the ones built into the core of Jira.
You will need to find an App that provides a similar function that can update custom fields (most Apps I know keep it as a single function and include the system fields alongside custom fields). Have a look in the marketplace for ScriptRunner, PowerScripts, Automation, Suite Utilities, Misc Workflow Extensions, etc
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