Also found it. Thank you!
The Power BI key is listed in Download Special Instructions for "SQL Server 2016 Enterprise Edition - Per Core" similar to below:
Running Power BI Report Server software also requires active Software Assurance. Please follow this link to access the software: https://go.microsoft.com/fwlink/?linkid=849940 and then use this product key to install: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
NOTE - Currently, the SP1 version of the same VLSC download does NOT include any mention of Power BI Report Server or a key.
We were able to grab the product key too. One question though. Could this product key be used just once ? We have a Development, Acceptance and Production environment. Wondering if the product key is applicable to multiple environments. If not, how do we go about it insalling in the three environments?
You can use your key for multiple production environments, provided you've licensed the required number of processor cores. For non-production environments (development, test, UAT, demo), you can choose Developer edition, which doesn't require a key at all.