The word you need here is 'in'. 'For half an hour' refers to a period of time. For example: 'There was a long queue at the airport check-in. I had to wait for half an hour.' 'In half an hour' means 'at a time which is half an hour from now'. For example, if it's 12.30 now, and your train leaves at 1.00, you would say that the train leaves 'in half an hour'. I hope that's clear now.

So, which one do you think is right? There's only one.