If you are in TurboTax Online, the screen Charitable Cash Contributions under CARES Act appears as the end of Deductions & Credits. First you have to pass the screen Based on what you told us, the standard deduction is best for you at the end of Deductions & Credits.

But you have to qualify for the standard deduction before the screen appears.  So you may want to change the value of one or more itemized deductions to $1 so that you qualify for the standard deduction.

Follow these steps: 

• At the top of the screen, click on Deductions & Credits.
• Scroll to the bottom of the page and click on Wrap up tax breaks.
• Continue through several screens.
• At the screen Based on what you told us, the standard deduction is best for you, click Continue.
• At the screen Any Charitable Cash Contributions? remove the dollar amount. 
• Continue through Federal Review to make sure that things look correct.
• Then restore your itemized deductions dollar amounts.

See also this TurboTax Help.

It turns out the reason it defaulted back to standard no matter what is because once the $300 charity credit you can take with the standard deduction was factored in, it actually made that the preferable filing option for federal, since my itemized deductions were only slightly above the standard deduction. It still kept itemized for state filing. It took awhile for the help files to explain that, but it's been clarified now.