Hi Christian,
Using Automation for JIRA, copying a field into the summary of an issue is actually not too hard with the use of smart-values.
In the summary you can reference the original issue and extract the value. Use a value like:
Your summary plus {{triggerIssue.fields.Custom Field Name}}
Simply replace Custom Field Name with the name of the field you want to copy.
This can be used in the summary of a sub-task using the "Create Sub-task action" or in the summary of an issue using the "Create Issue" action.
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