I'm not sure you need to adjust anything in TurboTax. The taxable amount of your distribution would be the amount in box 1 of your form 1099-R, less the amount in box 5, assuming you withdrew all of the money in the annuity. The tax that they withheld is entered on your tax return as a subtraction from your tax on the distribution.
To summarize, the amount of your distribution less the $100,000 that you contributed is taxed and that tax shows on your tax return. Then, the amount they withheld ($8,700) is also listed on the tax return and is subtracted from your tax. So, you are not being taxed on the $100,000.
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