Enter it as a positive number. If the result is a higher tax bill, double-check that you have actually entered your Foreign Source Income for 2024. You cannot use a carryback in a year where you have $0 foreign source income.
“Carryback” always applies to the Credit, not the liability. If you paid too much tax in 2025, you can carry that credit back to 2024 to lower your 2024 U.S. tax. The rule is “back one year, then forward ten years.” If you have extra credits in 2025, you start by applying them to 2024. If there’s still some left after that, the rest moves forward to 2026. You can’t carry credits back to 2023—the carryback only covers one year.
As far as making a Simplified Limitation Election" for AMT, simplified is recommended, especially if your foreign taxes are one category of income, such as the passive category. It tells the IRS: "Use my regular tax foreign income numbers to calculate my AMT foreign credit." You will make this election in your 2024 amended return, even if you didn't pay AMT in 2024. The election stays on your "permanent record" with the IRS for future years.
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