If you are referring to Box 18 of your W-2, then if Box 19 is blank, you can leave Box 18 blank as well. Box 18, 19, and 20 should all be blank.
However, if you have an amount in Box 19, you do need an amount in Box 18. If you know your local rate, say 1% for example, divide box 19 by the rate to get what to put in 18. So, if you have $150 in box 19, $150 divided by .001 equals $15,000, and that should have been reported in box 18.
