How to start second MySQL instance on boot on Ubuntu 20.04?

I have a second MySQL 8 instance running on Ubunut 20.04, started by this command:

mysqld_safe --defaults-file=/home/user/mysqld_replica.cnf & 

The config file contains all needed settings like port, socket and data dir. That works well but I can not figure out how to start the second instance on boot.

What would be the recommended way to start both instances at boot time?