I paid additional taxes in 2018, are these included in the federal tax withholding in my W2? Where can I enter this value in TurboTax?

What taxes did you pay?  Withholding from what?  Or did you send in estimated payments?

I asked my employer to deduct additional $50 out of my paycheck for additional taxes because I was worry owe taxes at the end of the year. So I assume this is addidtion payments went towards my federal income taxes
I’m employee who receives W2, Not self employed

Thank you so much.

Then it is included in the withholding in box 2 for federal and box 17 for any extra state.  This year the IRS reduced reduced the withholding tables so they took out less withholding but you had an extra amount, that was a good plan.  Just enter your W2.  

Thank you so much