Power BI Report Server License can be obtained by any of one
1 Purchased Power BI Premium
2 Purchased Software Assurance agreement
In your case it will be option 2
If you have a SQL Server Enterprise SA agreement, you can get your product key from the Volume Licensing Service Center. Look under the latest service pack, for the latest version of SQL Server. If you don't see it there, look under the RTM release of the latest SQL Server version.