I'm not certain, but at this point in time Online TurboTax may only be able to complete an out-of-country extension for military. At least that's what I see when I try to test it.
If mailing a paper Form 4868 (needs postmark by June 17), one can check the box that says "out of country". Save proof of mailing/tracking for your records as SteamTrain suggested.
Remember an extension is only an extension of time to file the return. It does not extend the time to pay any Federal taxes due last April.
