Per IRS Publication 3 Armed Forces' Tax Guide, on page 34:

Spouse in combat zone. If your spouse can't sign because of disease or injury and he or she tells you to sign, you can sign your spouse's name in the proper space on the return, followed by the words “by [your name], Husband (or Wife).” Be sure to sign your name in the space provided for your signature. Attach a dated statement, signed by you, to your return. The statement should include the form number of the return you are filing, the tax year, the reason your spouse couldn't sign, and that your spouse has agreed to your signing for him or her.

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