There are two places that you can enter the deduction. One is to enter it as a regular charitable donation under Deductions & Credits. If you don't enter it there, when you finish the Deductions & Credits section and click "Done with Deductions," a few screens later, after it tells you that you are getting the standard deduction, you will get a screen that asks whether you made cash charitable contributions up to $300.
Enter your cash donation in the Charitable Donations section. If you do not itemize your deductions the program will enter up $300 on Form 1040 Line 10b as an Adjustment to Income.
To enter charitable donations -
Or enter charitable donation in the Search box located in the upper right of the online program screen. Click on Jump to charitable donation