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In the case of a custom field, <field_name> has the form customfield_xxxxx, where xxxxx is the custom field numerical ID, as seen in the URL when editing the field. Or, starting with JIRA Misc Custom Fields v1.76.0 2 or JIRA Misc Workflow Extensions v. 3.7.0, you can also use the "real name" of the custom field (as shown in the list of custom fields).
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