Thank you for your answer. So, I have tried this and it doesn't work for me. This is a licensed product (Paid for) so not sure why this is the case. I will need to investigate further.
The epics do not appear in the list and the message I get is that "Portfolio for Jira must be licensed for the parent link to be available".
You'll want to assign the epic link not the parent link. parent link is a portfolio only field (as you discovered) for extending epics and other issue types with additional parent hierarchies.
I think that epic link dropdown is auto complete so you'll want to start typing in the epic name and it'll start showing you options.
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