You need a trigger which checks wether you are in the tank or not.. I assume you already have that trigger and are now stuck at the objective trigger. Also assuming that you have already made the briefing or there will be no objectives to be completed.
Place in the activation area:
"objective number" status "done"
If it's your first objective to complete, it's "1" status "done" and so on.
The unofficial command reference manual is very handy and it includes the objectives trigger.