You would need to get this information from your prior year tax return.
Your federal tax liability amount is found on Form 1040 (line 63), Form 1040A (line 39), and Form 1040EZ (line 12).
If you filed your taxes with TurboTax Online last year and need to print or download a copy to get this information, here is how:
https://ttlc.intuit.com/replies/3301525