There is this scriptrunner example to clone an issue in Jira as a workflow postfunction:
Is it possible to adapt it to clone an issue in another Jira instance?
It would not be an adaptation, but something completely different using the rest api and the RemoteIssueLinkService.
However as Zulfadli says I would look to existing plugins as there may be some custom field matching to be done which, although not tricky, is probably time-consuming and boring work.
If that plugin only provides a web-item to do the remote copy, you can probably work out how to script it using script runner.
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