The amount of your nontaxable combat pay should be shown on your Form W-2, Box 12, with Code Q.
To enter nontaxable combat pay for Form W-2, Box 12:
Per IRS Publication 3 Armed Forces' Tax Guide, page 19:
Earned Income Credit
For purposes of the earned income credit, earned income includes the following:
For purposes of the earned income credit, earned income does not include:
You can elect to include your nontaxable combat pay in earned income for the earned income credit. Electing to include nontaxable combat pay in earned income may increase or decrease your EIC. Figure the credit with and without your nontaxable combat pay before making the election. If you make the election, you must include in earned income all nontaxable combat pay you received.