On Screen 17 Dispositions, on the bottom of left column, click Schedule D. Enter the amount in the appropriate field and Lacerte will assign the code based on the input field you selected.

For example: (1) If you type an amount next to  Adjustment to basis if reported incorrectly on Form 1099-B, box 1e, Lacerte will assign code B. (2) If you type an amount next to District of Columbia Enterprise Zone, Lacerte will assign code X.