How can I input the amount deducted for federal income tax reported in box 2a from my CSA 1099-R?

Box 2a is the taxable amount for the 1099-R distribution.

 

The Federal Income tax withheld is in box 4 of the CSA 1099-R.

 

These amounts are entered as part of the 1099-R interview in TurboTax. Did I understand your question?

 

 

On the CSA-1099-R box 4 should have any federal income taxes withheld already entered.