Downgrade OS to version older than the OS on the Mac

I need to downgrade the OS on my MacBook Pro. My computer (latest 2018-model) is born with Mojave 10.14.2. I need to install Sierra 10.12.6. I work with Final Cut Pro 7, which is not working with OS later than 10.12.6.

I also have an iMac with 10.12.6 installed. I tried to make a TimeMachine backup from the iMac, for install on my new MacBook Pro. This was not possible. I then tried to download 10.12.6 on my iMac and created an external USB-installation disc for installation on the MacBook Pro. This was also not possible, because the OS is older than pre-installed on the Mac.

So I really need a solution more advanced than usual. I hope very much anyone can help.

Best regards

Kasper