At this time there is no way to place a monthly recurring trade.
However, you could set a series of Alerts to trigger an order at a designated date and time (in this case at the same time each month). To set up Alerts in Trader Workstation, right click on the symbol you wish to set the Alert for and left click on the "Alert" bell icon in the right click menu. Click the "Advanced Settings" button to choose a "Time" Alert. Click "Next" and a "Create Condition" window will populate where you can configure the time you wish to have your order triggered. Once configured, click Finish.
Under "Actions", click the Orders tab to trigger an order. In the "Orders" section, click the Add button all the way to the right to set up your order. Start by entering the symbol you wish to trade in the Financial Instrument field. Click enter and choose the proper product. Configure the order you wish to trigger. Once complete, click Activate at the bottom of the window. If you wish to set up a monthly recurring trade you must repeat this process for each month you wish to trigger an order.