Pretend that you received a 1099-B and enter the data that way.
To get to the correct page, do a Search (upper right) for 1099-b (lower case) and click on the jump-to link. Ignore any suggested search terms.
Even for a privately held company, you should be able to supply the necessary information.
This is for an ordinary stock sale, not an RSU, ESPP, or other employee stock plan. If the stock sale is one of those, come back to us.