macOS should install the Rosetta2 emulator automatically if you try and launch an Intel application. Our launcher forces FCP to launch as if it was an Intel Application. If it doesn't start FCP in Rosetta 2 mode then please try installing Rosetta2 using the command line here.
Another way to install Rosetta 2 on the Mac is by using the familiar softwareupdate command line tool.
This will launch the rosetta installer and you’ll have to agree to a license agreement, which I’m sure you’ll read completely and thoroughly as we all do every time we install anything on every device.
Then try and use our Rosetta2 launcher again, which you can get from here: