Click on Configure->Global Configuration menu.
A window comes up. Scroll to the Order section and expand it by clicking on it. Expand Order Defaults. Click on Fut. There is a check box called "Allow order to be activated , triggered, or filled outside of regular trading hours."
Cancel and Resubmit your order. If it is marketable, it will be filled. Otherwise it goes into the limit book. Beware that IB servers shut down at midnight and then start again, so be careful around that time.
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