See this post / answer that is about this exact same thing, nessus: CMD.exe Double Quote Error
It should work if you do the following:
waithidden cmd.exe /C ""C:\Program Files\Tenable\Nessus Agent\nessuscli.exe" agent link --key=KEY_REMOVED --host=abc123.com --port=8834"
If you check the bigfix client logs when the command was run, you should see what the client is trying to do.
In this case, you likely need the entire command to be wrapped in quotes so that the entire command is processed by CMD properly.
Since you are not redirecting the command output to a file, this might also work:
waithidden "C:\Program Files\Tenable\Nessus Agent\nessuscli.exe" agent link --key=KEY_REMOVED --host=abc123.com --port=8834