You don't even need to enter it, or leave the description blank or select "other" (or whatever the program's nearest choice to that is.)
Box 14, in the main, is simply "memo" information. It's a box in which your employer can tell you more about other dollar amounts on the W-2. In this case the employer is telling you how much is included in Box 1 of the W-2 for your personal use of a company car. So the dollar amount is already included in Box 1 (Gross salary or wages) and entering that amount again in Box 14 shouldn't affect anything because it's just "memo" information. Likewise, not entering that dollar amount in Box 14 won't affect anything.