Additional Medicare Tax Withholding is considered a payment for purposes of completing Form 2210.  From Form 8959 it goes to line 25(c) of the 1040, then becomes part of the total withholding on Line 25(d). Form 2210 instructions:  

Line 6
Include the taxes withheld shown on the following lines:
• Form 1040 or 1040-SR, line 25d