There is no setting within the Market Depth Trader tool or buttons which will allow you to trade outside of regular trading hours. However, what you can do is enable your orders to be activated, triggered, or filled outside of regular trading hours from the Order Presets. By doing this, your orders will automatically populate with "fill outside RTH" enabled when you submit the order from Market Depth Trader. Alternatively, you can edit each order individually once it is placed and enable the order to fill outside RTH.
To configure your Order Presets, click on the Edit menu option and select Global Configuration. On the left side, click the option for Presets. On the right side, the Presets section will load. The first section should be timing. Look for the box that says "Allow order to be activated, triggered, or filled outside of regular trading hours (if available)". Check this box, and select Apply and OK at the bottom of the screen. If you only want your order to be eligible to trigger during the pre-open session and not after hours, you can check the box that says "Allow order to be routed and executed during pre-open session".