You'll have to refer to the secret that you mentioned inside the pod definition.
If your're creating a secret with kubectl create secret docker-registry my-secret --docker-server=123.456.789.0... it should match exactly to what you're putting in your pod definition including the port number and in case it is secured then it be the same as the one in docker command line in systemd.
One more thing you need to make sure is that the secret and your pod should be in the same namespace otherwise they won't be able to communicate.