Contact the author directly. Explain your situation. They may just say it is unsupported.
If they do, it probably is best to find an alternative application. We can't give you any means of obtaining old activation codes here, as that would be breaking the license agreement of the author which is against the law. If you do install the newer Final Draft and need an upgrade pricing, the author would be able to work with you on that.
If you need to keep the older OS for other reasons, your 2015 can run up to Monterey on a separate partition or external hard drive for compatbility with newer applications. Dual booting is as simple as formatting the partition with Disk Utility APFS, or external hard drive that way. You'll want a minimum of 60 GB of free space on the partition, and the second partition with the OS should still remain at least 15% free after partitioning. If there is insufficient space, getting an external USB hard drive is a better choice.