I tried all types of orders on web and iOS app, it didn’t allow to input “-“ signCME has been sending out their "Advisory Notices" since April 3 regarding negative prices. I highly doubt a reputable firm like IB would have overlooked them.
https://www.cmegroup.com/notices/electronic-trading/2020/04/20200403.html#pageNumber=1
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If what you say is true, would you want to trust your hard-earned money with these slow firms?