Sql server Reporting Services activation with Sql server Core edition

One of my customer is trying to install Sql server reporting services 2019 and during installation customer gets 2 options; to select as free edition or activate through product key. However, customer states that he does not have a product key since he's using Sql server std core edition and hence unable to activate the SSRS 2019 product. Can anyone pls advise how SSRS 2019 can be activated if customer has Sql server std Core edition?

I've found this documentation for you:

Find the product key for SQL Server Reporting Services - SQL Server Reporting Services (SSRS) | Microsoft Docs

Hi Rogier,

Thank you sharing the link. I've go through and apparently customer couldn't find a key since it's sql server std core edition. Where can we find the key for Core edition?