Programmatically Setting Package Variables In Job Step Command Line

We are trying to start a server job running an SSIS package and supply some parameters to the package when we start the job using SMO.

What we have now is this:

string cmdLine = job.JobSteps[0].Command;

cmdLine += @" /SET PackageGetGroupRatingYear_Id.Variables[User::RatingId].Value;1";

cmdLine += @" /SET PackageGetGroupRatingYear_Id.Variables[User::GroupId].Value;1";

cmdLine += " /SET \Package.Variables[User::period].Value;"" + periodEndDate + """;

job.JobSteps[0].Command = cmdLine;

job.Start();

It appears that when the job is run, the modified command line is not used.

What is needed to supply runtime parameters to a job step when starting the job via SMO?

Thanks,