If you want an objective to be hidden, the best place to put the command is in your init.sqs.
When you want to make it appear, or when an objective is complete, the line of code usually goes in the On Activation line of a trigger.
You are currently undergoing a phase of learning about mission editing that we all went through: every time you discover something about how to make the mission work you find out that there is another stage you need to know to make the first things work .... it can be a little frustrating, but persevere - we all went through it.
And don't worry, you can ask as many questions as you like as long as you read the tutes to which we direct you.