You hide the objective, and then you activate it using a trigger.
In your init.sqs, let's say it's your first objective, you'll write:
"1" objstatus "Hidden"
Then, in the 'on activation' field in the trigger, you'll write:
"1" objstatus "Active"
That's should do it.
The objective is there all the time, just that they won't see it in the notebook, until you activate it.