You have to predefine all of your objectives in the briefing.html file. Then, you may hide the ones that you want to appear later by putting the following code in the init.sqs file:
"1" objStatus "HIDDEN"
Do this for each objective that you want to be hidden, with the number corresponding to the appropriate objective as you defined it in the briefing.html file. When you want the objective to appear, use the following code in your script, trigger, waypoint, or whatever you're using:
"1" objStatus "ACTIVE"
:cheers: