You will be reporting a 1099-B by going to Income >Sale of stock and can report your sale there. Since the cost basis was not known I suspect that the box for cost basis is blank. Input your calculated cost basis. Technically you could report 2 sales, one long term which would include the long term dividends added over the years and a separate sale for the short term dividend amount but that gets complicated and is probably trivial in terms of your total tax liability.
