Ideally, you contact your employer for a corrected W-2, or at least the box 18 number. Alternatively, Enter the same amount as line 16, unless you know it to be wrong. Box 16 and box 18 are usually the same*, unless you worked in more than 1 locality in 2024.
*Depending on where you are (e.g. Ohio), box 5 may be the correct amount, rather than box 16