It would be Box 2 of your W2. You could also find a reasonably close estimate if you have a pay stub from the end of the year. But, you only need this information if you want to estimate what taxes you might owe and actually pay them during the extension filing process. If you are expecting a refund or don't want to prepay an amount, then you can enter any sort of estimated amount in that box and just hit "Skip Payment" on the next screen. You should then be able to go on and finish the extension process.
