Need sidekick service to restart primary service -- how?

I have a stack which does mail. The primary service runs Postfix and all the rest. The mail is secured via LetsEncrypt as maintained by Traefik. I have the ability to extract the mail server’s certificate and private key from Traefik’s JSON file, and I’m putting that into a sidekick service. My goal is to restart the primary service when the cert and key are updated. How can I restart the primary service from a sidekick?