Originally Posted by HiAperture

No way to do what you are saying, book a round trip but only tell the airline when and where you want to go on one leg, you will have to tell them both legs at the same time. You will have to look at the fare rules for your trip, usually 6 months is the max. You can always just pick something random and far out in dates, then pay to change the second leg later, but there will be change fees and fare differences at that time.