Quote from jimrockford:
I would report the EFP on Schedule D, as a constructive sale of an appreciated financial position. I would describe the EFP in terms of its stock leg, without specifically mentioning the EFP or the SSF. I would give both the date of acquisition of the stock, and the date of sale, as the date on which I sold the EFP. I would give the acquisition price as the price I paid for the stock leg. I would give the the sale price as the price for which I sold the SSF leg, less commissions. I would also report both the stock leg and the SSF leg of the EFP as a straddle on IRS Form 6781.
I am not a tax expert. If you do what I would do, and it turns out that I am wrong, then you could be in big trouble.
I think that IB should prepare guidance for its customers, responsive to your type of question.