Hi @Wojtek Deka , Welcome to the Atlassian Community!
The Field is named "Parent Link". But you can stick with customfield ID. Here's how to find out the ID for the Field in your instance:
https://<yoursite>.atlassian.net/secure/admin/ViewCustomFields.jspa?page=1&searchFilter=parent
This should filter for Field parent link.
Now click on the three points to the right of the field and hover over "View field information".
Your browser should preview a link like this:
https://<yoursite>.atlassian.net/secure/admin/ConfigureCustomField!default.jspa?customFieldId=10018
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