Deductions & credits

1. Click Deductions and Credits
2. Go through the screens until you come to Wrap Up Tax Breaks
   If the standard deduction is chosen you will see a screen that asks Any Charitable Contributions?
3. Make sure that $300 or correct amount  is filled in and then click Continue
4. Click Lets Keep Going

The "trick" is to get it to go through the charitable deductions again and then TurboTax understand that you are taking a standard deduction and qualify for the $300 charitable cash deduction.