This article shows how to copy an issue key from JIRA instance to a summary field in the twin issue on another JIRA instance.
You can filter local issues by the remote key using JQL search.
Add the code below to copy the remote issue link to the previously created custom field.
Congratulations! Now you know how to copy an issue key from one instance to the summary fields on another instance.
Here is an example how it looks once you synchronize the issue.