In TurboTax Premier, on the screen where you enter the basic information about the sale, enter all the information, then click the button that says "I'll enter additional info on my own." You might have to scroll down a little to see the button. That takes you to a screen that has additional boxes, including box 1g.
Actually, in my case (TurboTax Premier), there is no place that says "I'll enter additional info on my own." for 1099-B. That is something I was looking for, since some other forms has it.
Anyway, I was able to do it by selecting "I need to adjust my total cost basis", entering the amount from box 1g and selecting "W (nondeductible loss from a wash sale)".
I don't know why mine is different, perhaps because I selected to enter the summery than all transactions. Anyway, I think TurboTax can make it simple by giving the option to enter what is in box 1g on the first page itself.