where do I enter info about my RMD?

You enter the withholding in box 4 when you enter the 1099R.  It’s there, I guess it’s easy to miss.  

Enter a 1099R under
Federal Taxes
Wages & Income
Then scroll way down to Retirement Plans and Social Security
Then IRA, 401(k), Pension Plan Withdrawals (1099-R) – Click Start

You didn't actually pay the tax.  You had taxes withheld like from your paycheck. You still have to enter the 1099R and whole gross original amount (before taxes were withheld) with your other income to figure out the total tax (and it may put you into a higher tax bracket) and then the withholding is subtracted from the total tax to figure your refund or tax due.  The gross amount shows up,on 1040 line 4a or 5a and the taxable amount on 4b/5b. The withholding will show up on 1040 line 25b.

You should have form 1099-R documenting your withdrawal and containing the amount of tax withheld. Enter that when you report that form.