Was this for a RMD? How was it sent to the charity? Did the IRA send it directly to it? What code or codes are in box 7?
If you got the full amount and you sent half to charity the full amount is taxable and it is a Schedule A deduction. @dmertz
You put in the whole thing
If your age was over 70.5, then there is a followup question on one of the pages that follows the main form, that lets you indicate how much went to the charity. But the $$ had to be directly transferred to the charity (with some exceptions). That will reduce the amount that is reported on line 4b of the form 1040
If you were not over age 70.5 during 2019, then it is all taxable, but you enter the $$ you donated to the charity in the charity donations section.