Claiming automatic extension

I am currently serving in a designated combat zone (since mid December). I expect to leave the combat zone mid-June. I know that I am eligible for an automatic 180 day extension but I'm not sure how to claim it.

I imagine that if I simply file after the deadline I will be slapped with a penalty. Do I attach a statement explaining my extension or is there a form for this. Or does the IRS actually receive information from the Army about my dates of service. Pub 3 explains all extensions but doesn't explain how to use it.