If you mean "bonus depreciation," this special allowance is calculated when you report a new asset for your business. If the asset qualifies, TurboTax will present various options for writing off the cost of the asset, including Special "Bonus" Depreciation.
To enter a new asset, go to your business summary page and look for Assets/Depreciation. Start/Update and follow the interview.
Additional information: IRS Pub 946: Claiming the Special Depreciation Allowance
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