If you are taking the standard deduction and already have more than $300 in charitable cash donations, then you are correct that adding another donation is not going to change your refund or amount owed. If you are taking itemized deductions, then there would be an impact on your final amounts owed or due.
You would enter the donation in TurboTax using these steps"
Federal
Deductions and Credits
Charitable Donations
For 2020 you can claim up to $300 in cash donations to charity and that amount is added to your standard deduction.
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