You need to make a request to POST /trade/v2/orders specifying order details. In the request body you need to specify the flag IsForceOpen as True. Additionally you need to allow these types of orders for your client, so you need to go to https://www.saxotrader.com/ and enable Force Open. Otherwise you will encounter an ForceOpenNotAllowed error when attempting to place orders. To enable force open follow the steps below.
Step 1 - Click Settings.
Step 2 - Select Platform.
Step 3 - Check "Enable force open of positions by default".
Now you should be able to place orders with Force Open.