SMJobBless is for the Privileged Helper tool, but what should I do if I want the startd plugin to be copied to /Users/<user>/Library/LaunchAgents . I don't want to enter a user password to install the helper tool.
/Users/<user>/Library/LaunchAgents
Your application should already be able to create processes launchctl and have write access to /Users/<user>/Library/LaunchAgents , so can't you just do this directly instead of trying to use it SMJobBless() ?
launchctl
SMJobBless()